Gender, Nature, and Nurture - Richard A. Lippa 2014

What Do Meta-Analyses Tell Us about Sex Differences?
What's the Difference Anyway?

It is time now to turn to evidence about sex differences. To make our discussion manageable, the evidence will be organized into the following topics: personality, risk-taking, social behavior, sexuality and mating preferences, interests, intelligence and cognitive abilities, and physical traits (e.g., grip strength and throw velocity). Finally, we shall turn to sex differences that have not been well summarized by meta-analyses: in mental illness, emotional experience, self-concept, and childhood friendships and interaction styles.

Are There Sex Differences in Personality?

Personality traits can be defined as internal factors, partly determined by experience and partly determined by heredity and physiology, that cause individuals' characteristic patterns of thought, feeling, and behavior (Funder, 1997). Personality traits lead people to show consistencies in their behavior, both over time and across settings. For example, an extraverted person will probably continue to be extraverted a month from now (consistency over time). Furthermore, an extraverted person likely shows his or her extraversion in many different situations: at work, at parties, and at home with family members (consistency over settings). Trait theories typically assume that people vary along a measurable dimension. for example, the dimension of introversion-extraversion.

What are the key trait dimensions of personality, and how many different personality traits are there? The answer to this question depends in part on whether you focus on very broad or very specific traits. Over the past two decades, personality psychologists have reached a consensus that, at the broadest level of description, human personality can be characterized by five traits, which are termed the Big Five (Wiggins, 1996). The Big Five traits are extroversion, agreeableness, conscientiousness, neuroticism, and openness to experience.

Although these labels may seem straightforward, the traits they refer to are sometimes broader than their labels might suggest, and each Big Five trait comprises a number of sub-traits or facets. People who are highly extraverted, for example, are sociable, bold, assertive, spontaneous, cheerful, and energetic, and introverted people are just the opposite. Agreeable people are warm, kind, polite, friendly, and good-natured, whereas disagreeable people are cold, irritable, hostile, vindictive, and unfriendly. Conscientious people are careful, serious, and responsible; they manage their impulses well and abide by social rules and norms. In contrast, people who are low on conscientiousness are unreliable. They have difficulty controlling their impulses, and they sometimes act out and break social rules. People who are high on neuroticism are nervous, depressed, tense, and suffer from low self-esteem, whereas people who are low on neuroticism are calm, well adjusted, self-assured, and confident. Thus, people who are high on neuroticism suffer from many negative emotions, and indeed, another label for neuroticism is negative affectivity or negative emotionality. Finally, people who are high on openness to experience are imaginative, curious, creative, and liberal. They take pleasure in intellectual and artistic experiences and love variety in food, travel, friends, and acquaintances. People who are low on openness tend to be closed-minded, conventional, and set in their ways. They do not value introspection or aesthetic experiences.

Do men and women differ on the Big Five personality traits? Yale psychologist Alan Feingold (1994) conducted a meta-analysis to answer this question. Much of the data Feingold reviewed came from large samples assessed to develop norms for standardized personality tests. Thus the data were not collected specifically for the purpose of studying sex differences.

Feingold found that the traits showing the largest sex differences were facets of extroversion and agreeableness. The extroversion component that showed the largest sex difference was assertiveness (d = 0.50, with men more assertive than women). I he Agreeableness component that showed the largest sex difference was "tender-mindedness" (d = 0.97, with women more tender-minded than men). In terms of Cohen's guidelines, these sex differences are moderate and large, respectively.

Feingold's meta-analyis found a modest gender difference in anxiety, a neuroticism facet (d = 0.28, with women more anxious than men). There were negligible gender differences in conscientiousness and openness to experience (although it is worth noting thai Feingold did not examine all possible facets of these traits). Feingold also summarized the results of studies on self-esteem, and although he found a slight difference favoring men (d = 0.16), this difference was small and not of great practical significance. A more recent meta-analysis similarly found a small sex difference in self-esteem favoring men (d = 0.21), and this sex difference proved to be largest in studies of late adolescents (d = 0.33; Kling, Hyde, Showers, & Buswell, 1999).

One recent study analyzed sex differences in Big Five personality traits that were assessed in more than 23,000 people from 26 cultures (Costa, Terracciano, & McCrae, 2001). Women consistently scored higher than men on many facets of neuroticism and agreeableness. Men scored higher than women on assertiveness and excitement seeking (Extraversion facets), competence (a facet of conscientiousness), and fantasy and openness to ideas (facets of openness). Costa and his colleagues further found that sex differences in personality tended to be stronger in economically advanced countries with liberal gender ideologies (e.g., the United States, and European countries) than they were in less economically advanced countries with more traditional gender roles (Asian, African, and Latin American countries). To explain these unexpected findings, they hypothesized that in countries with strong, traditional gender roles (e.g., countries that do not offer many educational or work opportunities to women), men and women attribute their differing behaviors to their societies' obviously powerful gender roles. However, in countries with weaker gender roles, which do not so obviously constrain their behavior, men and women attribute their differing behaviors more to internal factors (e.g., personality traits). Whatever the proper explanation for these cross-cultural variations, Costa, Terracciano, and McCrae's findings generally replicated the overall pattern of sex differences reported in Feingold's earlier metaanalyis.

Another recent study measured personality traits in almost all the 18-year-oid boys and girls who had been born during a 1-year period in Dunedin. New Zealand (M of fit, Caspi, Rutter, & Silva, 2001). As a result, this study was able to examine sex differences in personality in a sample that was representative of an entire community. Participants

completed the Multidimensional Personality Questionnaire (Tellegen, 1982), which measures traits that overlap substantially with Big Five traits. The observed sex differences in personality are summarized in Table 1.1. Note that women were much higher than men on harm avoidance, and they were moderately higher on stress reaction, social closeness, and self-control. In contrast, men were much higher than women on aggression, and they were also somewhat higher on alienation and achievement. These findings corroborate Big Five studies showing that women tend to exceed men on certain facets of neuroticism and agreeableness, and they also suggest that women exceed men on certain kinds of conscientiousness (or self-control). The Dunedin study is doubly interesting because it found that observed sex differences in personality strongly predicted sex differences in participants' antisocial behaviors (e.g., behaviors such as lying, stealing, delinquency, drug and alcohol abuse, sexual acting out, and criminal behavior). This indicates that the observed sex differences in personality were for real, in the sense that they predicted significant real-life outcomes. Thus it is unlikely that the sex differences in self-reported personality resulted solely from response sets, which might occur if men and women were simply responding to personality questionnaires in gender stereotypic ways.

There are two additional traits worth mentioning in relation to sex differences in personality: authoritarianism and social dominance orientation. Both of these personality traits are related to prejudice. Authoritarianism refers to the degree to which people defer to authority, follow traditional societal norms and conventions, and feel hostility toward people who are seen at outsiders (Adorno, Frenkel-Brunswik, Levinson, & Sanford, 1950; Altemeyer, 1981, 1988, 1998). Social dominance orientation refers to the degree to which people believe that some groups are better than others (Pratto, Sidanius. Stallworth, & Malle, 1994). People who are high on social dominance view the social world in terms of "haves" and "have nots," and they think that inequality is "the way things should be."

Are there sex differences in authoritarianism and social dominance orientation? Although there have been no meta-analyses on the topic, recent studies find that men and women do not differ much on authoritarianism (Altemeyer. 1988). In contrast, men and women do differ in social dominance, with men scoring higher than women (Lippa & Arad, 1999; Sidanius. Pratto, & Bobo, 1994). The d value for sex differences in social dominance orientation (based on my own summary of a number of recent studies) is around 0.6. Largely because of this difference, men tend to hold somewhat more prejudiced attitudes toward minority groups than women do (Altemeyer, 1998).

Are There Sex Differences in Boys' and Girls' Activity Levels?

Although most meta-analyses have focused on sex differences in adult personality, there may also be sex differences in aspects of infant and child temperament, which may be precursors to adult personality. Perhaps the best documented finding is that boys, on average, have higher activity levels than girls (Campbell & Eaton, 1999). An early meta-analysis of 14 studies of infant activity level found a mean sex difference of d = 0.29. However, critics argued that this finding was suspect because different studies used nonequivalent measures of activity (e.g., parents' reports, direct observations of movement, data from actometers— devices strapped to babies' limbs that objectively measure movements); furthermore, some studies observed infants' movements over shorter time periods than others (Martin, Wisenbaker, Baker, & Huttunen, 1997)

To address these concerns, University of Manitoba psychologists Darren Campbell and Warren Eaton (1999) conducted a more complete and up-to-date meta-analysis of 46 studies. They found that sex differences in infant activity tend to be larger in studies using direct observations and actometers (d = 0,20 and 0.21, respectively) than in studies using subjective parent ratings (d = 0.09). Thus, it appears unlikely that reported sex differences in babies' activity levels result from an "eye of the beholder" effect, which might occur if the stereotypes parents hold about boys and girls influence their ratings of their babies' activity levels. Furthermore, Campbell and Eaton found that although sex differences in infant activity levels tended to be larger when infants were observed for longer time periods (i.e., in studies that obtain the most reliable and trait-like measures of activity levels), studies that observed babies for short periods of time also found significant sex differences. Finally, sex differences in infants' activity levels did not depend on babies' ages (which ranged from 0 to 12 months). That newborns show sex differences in activity levels is interesting because little or no learning has taken place in newborns.

Sex differences in activity levels grow larger throughout childhood, to a mean d value of 0.44 in early childhood and 0.64 in middle childhood. These differences are socially significant because they are likely linked to later sex differences in personality (e.g., in aggressiveness, in facets of extraversion) and to sex differences in childhood and adult behavior problems (e.g., attention deficit disorder and conduct disorders). Furthermore, sex differences in children's activity levels may contribute to childhood sex segregation, which is the tendency for boys to associate mostly with other boys and for girls to associate mostly with other girls (Pellegrini, 2004). Apparently, active, high-energy boys find other boys most fun to play with, whereas more sedate and calm girls find other girls most rewarding to play with.

Are There Sex Differences in Risk-Taking?

Three University of Maryland researchers, James Byrnes, David Miller, and William Schafer (1999), conducted a meta-analysis that summarized 150 studies on sex differences in risk-taking. Some of these studies measured risk-taking via self-reports and others observed actual risk-taking behaviors (Table 1.2). In general, men proved to take more risks than women did (d = 0.13), but this difference is quite small. A d value of 0.13 implies that 45% of women take more risks than the average man does, and 55% of men take more risks than the average woman does.

An examination of Table 1.2 shows, however, that sex differences were larger for some kinds of risk-taking than for others. For example, men reported engaging in riskier driving practices than women did (d = 0.29). This value implies 61% of men reported riskier driving practices than the average woman did. Men also took greater risks than women in exposing themselves to danger in experiments, to intellectual risks, and to higher stakes in games of physical skill. The average d value for these sex differences was about 0.4, Thus, about 66% of men took greater risks in these sorts of tasks than the average woman did. Byrnes, Miller, and Schafer (1999) reported that sex differences in risk-taking tend to decrease with age. They also noted that sex differences in risk-taking have decreased somewhat in recent years.

Are There Sex Differences in Social Behaviors?

Social psychologists study behaviors such as aggression, helping, conformity, susceptibility to persuasion, leadership, and group behavior. Are there sex differences in these sorts of social behaviors?

Aggression.

As noted before, two meta-analyses in the 1980s of experimental studies of aggression found moderate sex difference favoring males, with d statistics ranging from 0.29 to 0.50. In a more recent meta-analysis, which included some newer studies, Ann Bet tencourt and Norman Miller (1996) found a mean gender difference of d = 0.23. This meta-analysis, like the earlier one by Eagly and Steffen (1986), included only experimental studies of adolescents and adults, and its results were quite similar to Eagly and Steffen's.

Bettencourt and Miller s meta-analysis further showed that sex deferences in aggression were larger in experimental studies of unprovoked aggression (d = 0.43) than in studies of provoked aggression (d = 0.06). (Provocations include insults, physical attacks, and frustration.) However, specific kinds of provocation did lead to sex differences in aggression. The kinds most likely to goad men to be more aggressive than women were physical attacks (d = 0.48) and insults about one's intelligence (d = 0.59).

By focusing only on experimental studies, however, the meta-analyses just summarized may have underestimated sex difference in aggression. After all, experimental studies often use somewhat artificial measures of aggression, and their participants tend to be college students, who may be on good behavior when serving as guinea pigs in university laboratories, To address these limitations, British psychologist John Archer (Archer & Mehdikhani, 2004) conducted a meta-analysis of nonexperimental studies that collected measures of real-life aggression (e.g., self-reports, peer reports, and teacher reports of aggression, as well as direct observations of aggression). Unlike previous meta-analyses, this review included many studies that were conducted outside of North America.

As noted earlier, overall sex differences in aggression proved to be larger in Archer's meta-analysis than in previous meta-analyses (d ranged from 0.42 for self-reports and teacher reports to 0.57 for peer reports), and when Archer restricted his meta-analysis just to measures of physical aggression, he found that some sex differences were larger still (e.g., the d for peer-reported aggression increased to 0.84, which implies that 80% of boys are rated by their peers to be more physically aggressive than the average girl is). Two additional findings from Archer's meta-analysis are worth noting. First, sex differences in aggression tended to decrease with age; peak values often occurred during young adulthood (e.g., the sex difference in self-reported physical aggression reached a peak value of d = 0.66 at ages 18 to 22). And although sex differences in aggression varied somewhat across nations and across world regions, males were nonetheless more aggressive than females in most countries and regions surveyed.

Only one kind of aggression showed a sex difference favoring females. Archer termed it indirect aggression, and others have termed it relational aggression: hurting others by ostracizing them from social groups and gossiping and spreading malicious rumors about them. However, this sex difference was relatively small, somewhat variable across measures and studies, and it tended to be most pronounced during adolescence. Considerable recent research has suggested that girls are more likely than boys to use and to be the victims of indirect and relational sorts of aggression (Crick & Nelson, 2002; Simmons, 2002).

Helping Behavior.

Social stereotypes hold that women are nicer and more nurturant than men (Eagly, Mladinic, & Otto, 1991). But are women truly more helpful? This may depend in part on the kind of helping studied. Some kinds of helping, such as giving money to charities and making soup for a sick friend, are common in everyday life and not risky to the helper. Other kinds of helping, such as running into a burning house to rescue a child or jumping into icy water to save a drowning victim, are rare, dramatic, and very risky. Social psychologists have tended to study risky and dramatic forms of helping—termed emergency interventions— more than commonplace, everyday kinds of helping, and this may have biased their findings somewhat in favor of finding men more helpful than women.

You may recall from our discussion of personality that women report being more "tender-minded" than men. A similar finding comes from studies of self-report measures of empathy. Women report that they are more empathetic than men are (d = 0.27; Eisenburg & Lennon, 1983). However, a meta-analysis of 182 studies that actually observed the helping behaviors of men and women in both laboratory and field settings found men, on average, to be a bit more helpful than women (d = 0.34; Eagly & Crowley, 1986).

Sex differences in helping varied considerably across studies, however, Eagly and Crowley (1986) observed that men were more helpful than women, particularly when they were being observed by others (d = 0.74) and when the person they assisted did not directly request help (d = 0.55). Furthermore, men were more helpful to women in need than they were to men in need. Putting these findings together, it seems that men are more helpful than women particularly in public settings and when assisting women. A flattering interpretation is that men wish to be chivalrous. A more cynical interpretation is that men wish to look heroic before a public audience, especially if that audience consists of women. Men's tendency to be more helpful than women when help is not directly requested may reflect men's greater assertiveness and perhaps also, their greater intrusiveness.

University of California psychologist Shelley Taylor (2002) argued that women tend to others more than men do in a variety of ways. Women nurture children more than men do, they seek out and give social support in friendship networks more than men do, and they care for sick friends and family members more than men do. One finding that supports Taylor's conclusion that women are better "tenders" is that marriage brings many more psychological and health benefits to men than it does to women (Berkman & Syme, 1979; Litwak & Messeri, 1989). Apparently, having a wife is health-promoting, but having a husband is not necessarily. Because women assume more of the burden of tending for others, having an ill spouse takes more of a psychological and physical toll on wives than it does on husbands (Revenson, 2003). Thus, everyday tending—giving emotional support, caring for the ill, nurturing children, making sure others get adequate food, sleep, and health care—is a kind of helping behavior that very likely shows a sex difference favoring women.

Moral Behavior.

Humans differ from lower animals in that they (sometimes) follow moral rules. Moral behavior depends in part on internal factors such as individuals' moral beliefs and principles and in part on external factors such as social pressures and incentives to avoid evil and do good. Do males and females differ in the way they think about moral issues, and do they, on average, differ in the degree to which they give in to moral temptation?

In her classic book, In a Different Voice, Harvard psychologist Carol Gilligan (1982) proposed that males' moral reasoning may be based more on abstract principles of justice, and females' moral reasoning may be based more on conceptions of relatedness and caring for others. To test the evidence for this assertion, Sara jaffee and Janet Shibley Hyde (2000) conducted a meta-analysis of 113 studies on moral reasoning. They found that women showed slightly more of a tendency to use a care orientation in their moral thinking than men do (d = 0.28), whereas men showed slightly more of a tendency to use a justice orientation than women do (d = 0.19). Although these differences are small, sex differences in moral orientation could have cumulative effects in real-life settings, for example, if mothers and fathers repeatedly evaluate moral lapses in their children somewhat differently and if female and male jurors evaluate the criminal responsibility of defendants differently in thousands of trials.

Some studies have tried to assess directly how much men and women (and boys and girls) engage in specific types of "immoral" behavior, such as cheating on tests and engaging in forbidden activities. Bowling Green State University psychologist Irwin Silverman (2003a) conducted a metaanalysis of the results of 98 studies of various kinds of moral transgressions. He found that there were no significant differences in males' and females' levels of cheating on tests. However, girls were somewhat better than boys at resisting the temptation to engage in forbidden activities (e.g., playing with forbidden toys, touching forbidden objects, or eating forbidden foods); d ranged from 0.27 to 0.41 depending on the statistical methods used. Silverman hypothesized that resisting the temptation to engage in a forbidden activity (unlike deciding not to cheat) requires children to control their impulses in the presence of an immediate reward (e.g., a fun toy, a tasty piece of food). Thus girls may be particularly better than boys at inhibiting their impulses in the immediate presence of vivid and tempting rewards (see also, Bjorkiund & Kipp. 1996).

In another meta-analysis, Silverman (2003b) summarized studies on delay of gratification, which ask people to choose between an immediate small reward or a delayed larger reward (e.g., a child might be asked to choose between a small piece of chocolate right now versus a whole candy bar in 15 minutes). After analyzing the most reliable measures of people's ability to delay gratification, Silverman found a slight difference favoring females (d = 0.19), which did not depend on the age of participants. Although small, this difference could have important, cumulative real-life consequences, for example, if women are better than men at saving for a rainy day or at postponing pleasurable but possibly dangerous sexual activities.

Conformity and Social Influence.

Conformity refers a person's tendency to shift his or her opinions to be more like those of a group, presumably because of pressure from the group. Think of teenagers who dress like their friends, or church members who espouse the same religious beliefs as other members of their congregation. Solomon Asch (1956) conducted classic early studies of conformity in which college students were asked to make obvious perceptual judgments, such as judging which of three lines was equal in length to a fourth line. On some judgment trials, the answer that students believed to be correct was openly contradicted by a unanimous group of peers. Asch observed how often students would cave in and go along with the crowd when other peers openly disagreed with their judgments. Since Asch's time, hundreds of additional studies have investigated when and why people conform.

Two meta-analyses have examined sex differences in Asch-type conformity experiments (Becker, 1986; Eagiy & Carli, 1981). Their findings were quite similar. On average, women conform a bit more than men do (d = 0.32 and 0.28, respectively). Sex differences in conformity are strongest in face-to-face settings, such as those employed in Asch's studies, when subjects are in the direct presence of peers who exert pressure on them to conform. Becker (1986) and Eagly and Carli (1981) also summarized evidence on sex differences in people's degree of attitude change after hearing or reading persuasive messages. Women proved to be slightly more persuaded on average than men were (d = 0.16 and d = 0.11).

Group Behavior.

For over half a century social psychologists have studied behavior in various kinds of groups: therapy groups, work groups, juries, and other kinds of decision-making groups. Small group research has identified two basic kinds of group behaviors: social-emotional behaviors and task-oriented behaviors. Social-emotional behaviors, such as telling a joke to relieve group tension or praising another group member who does a good job, are focused on maintaining personal relationships in groups. Task-oriented behaviors, such as offering information or asking for solutions to problems, are focused on achieving the work goals of the group.

Do men and women differ in the amount of social-emotional and task-oriented behaviors they show in groups? In the early 1980s, two meta-analyses examined this question (Anderson & Blanchard, 1982; Carli, 1982). Both found similar results. On average, men engage in more task-oriented group behaviors than women do (d = 0.59 in Carli's metaanalysis), and women engage in more social-emotional group behaviors than men do (again, d = 0.59). This implies that 72% of men engage in more task-oriented group behaviors than the average woman does, and similarly, 72% of women engage in more social-emotional group behaviors than the average man does.

Other meta-analyses nave studied sex differences in leadership. According to a meta-analysis by Alice Eagly and Steven Karau (1991), men are somewhat more likely than women to emerge as leaders of unstructured laboratory groups (d = 0.32). Group leaders can be experts in either social-emotionai behaviors or task-oriented behaviors, or sometimes both. The social-emotional leader tends to be liked, has good people skills, and excels at reducing group tensions and managing group emotions. The task-oriented leader is hard-nosed and focuses on getting the job done and achieving group goals. Eagly and Karau (1991) found that men are more likely than women to emerge as task leaders of groups (d = 0.41), whereas women are more likely than men to emerge as social-emotional leaders (d = 0.18). In another meta-analysis Alice Eagly and Blair Johnson (1990) found that women on average show a more democratic leadership style in groups, whereas men show a more autocratic style (d = 0.22).

In a still more recent meta-analysis, Alice Eagly, Mary Johannesen-Schmidt, and Marloes van Engen (2003) examined sex differences in three styles of leadership that have been much studied over the past two decades: transformational leadership, which involves innovative and inspirational leadership that gains the trust, confidence, and admiration of group members; transactional leadership, which involves managing group members through the use of instructions, punishments, and rewards; and laissez-faire leadership, which occurs when leaders do not manage group members much and instead let them do their own thing. On average, women reported a slight tendency to use transformational leadership styles more than men did (d = 0.10). Sex differences were mixed for various kinds of transactional leadership; women were slightly more likely than men to use contingent rewards as a method of guiding and motivating subordinates and followers (d = 0.13), whereas men were more likely than women to focus on subordinates' mistakes as a leadership strategy (d = 0.12) or to intervene only when subordinates' were "making a mess of things" (d = 0.27). Finally, men were more likely than women to report that they used laissez-faire leadership strategies (d = 0.16). Although none of these differences is large, Eagly and her colleagues argued that, on average, women may have more effective leadership styles than men do because the styles of leadership that are more characteristic of women are exactly those that tend to predict actual leadership effectiveness.

Another important kind of group behavior is negotiation. In a metaanalysis of 21 studies, Aiice Stuhlmacher and Amy Walters (1999) found that men on average achieved slightly better outcomes in negotiations than women did (d = 0.09). The difference favoring men was strongest in studies in which the negotiator had a high degree of power (d = 0.25) and in studies in which the negotiation was a zero sum game, in which only one of the negotiators could come away with positive outcomes (d = 0.20). Although sex differences in negotiation appear to be small, Stuhlmacher and Walters argued that the cumulative effects of such differences could be larger. For example, over the course of many years, employees may negotiate salaries, working conditions, and promotions many times. As a result, small negotiating advantages favoring men could build up with repetition, and this cumulative effect could contribute to the greater number of men than women found in positions of power in government and business organizations.

Are There Sex Differences in Nonverbal Behavior and Nonverbal Perceptiveness?

People convey a huge amount of information to one another, both intentionally and unintentionally, through behaviors such as eye contact, smiling, facial expressions, speech intonation, gestures, and the use of personal space. Researchers have studied the nonverbal behaviors people display to one another and people's ability to read one another's facial expressions and body language.

Northeastern University psychologist Judith Hail (1984) conducted a set of classic meta-analyses that summarized sex differences in nonverbal behavior and in people's ability to decode (i.e., to understand or read) nonverbal behaviors. Table 1.3 presents some of her findings. In general, women are better than men at decoding nonverbal information (d = 0.43). A difference of this magnitude implies that 67% of women are better at judging nonverbal information than the average man is. Women are aiso better than men at posing emotions with their facial expressions (d = 0.52).

Men and women differ in specific kinds of nonverbal behavior. For example, women smile more than men do in social settings (d = 0.63), according to Hall's meta-analysis. A more recent meta-analysis of 162 studies found a smaller mean difference (d = 0.41) in smiling between females (women and adolescent girls) and males (men and adolescent boys) (LaFrance, Hecht, & Paluck, 2003). This value implies that 66% of females smile more than the average male does. Furthermore, LaFrance, Hecht, and Paluck's meta-analysis showed that sex differences in smiling were very sensitive to situational factors. For example, differences tended to be larger when participants knew they were being observed, when fewer other people were present, when participants were trying to become acquainted with others, and when participants had little or no prior acquaintance with their interaction partners. Sex differences in smiling also varied across ethnic and nationality groups and they were strongly related to age, with d values of 0.56 for teenagers, 0.45 for young adults, 0.30 for middle-aged adults, and 0.11 for seniors.

Smiling is just one of many nonverbal behaviors, and Hall's (1984) meta-analysis examined sex differences in many other nonverbal behaviors as well. Woman engage in more eye contact with others than men do (d = 0.68). Men maintain more personal space (i.e., physical distance) between themselves and others, both when approached by others (d = 0.56) and when approaching others (d = 0.95). A d value of 0.95 implies that 8356 of men maintain more distance from others than the average woman does. Men are more restless (d = 0.72) and expansive (d = 1.04) in their body movements than women are. Thus 85% of men show more expansive movements and gestures than the average woman does.

Women tend to be more facially expressive than men (d = 1.01); this means that 84% of women are more facially expressive than the average man is. Women also tend to have more expressive gestures than men (d = 0.58). Finally, men show more speech errors (e.g., stammers and stutters) than women do (d = 0.70). Similarly, men show more filled pauses (e.g., "ah," "um." and "er") in their speech than women do (d = 1.19). Many of the sex differences observed in nonverbal behaviors are quite large, and recent meta-analyses that have updated some of Hall's (1984) earlier ones have generally found similarly sized sex differences (Hall, Carter, & Horgan, 2000).

Are There Sex Differences in Sexuality?

Sexual relations constitute perhaps our most intimate form of social behavior. Mary Beth Oliver and Janet Shibley Hyde (1993) conducted a meta-analysis of 177 studies that investigated sex differences in various kinds of sexual behaviors and attitudes. Some of their results are presented in Table 1.4.

Here's a summary: Men hold more sexually permissive attitudes than women do (d = 0.57), and they hold more positive attitudes toward casual sexual intercourse than women do (d = 0.81). This implies that 79% of men regard casual intercourse more positively than the average woman does. Men report masturbating much more than women do (d = 0.96). A difference of this magnitude implies that 83% of men masturbate more than the average woman does.

Oliver and Hyde also reported a number of smaller sex differences. Men hold more positive attitudes than women do toward engaging in sexual intercourse in the context of committed relationships (d = 0.49) and in the context of marital engagements (d = 0.43). Women report more fear and guilt about sex than men do (d = 0.35). Men report engaging in sexual intercourse more than women do (d = 0.33), engaging in sexual intercourse at an earlier age than women do (d = 0.38), and having sex with a greater number of sexual partners than women do (d = 0,25). Finally, men report a higher incidence of homosexual behavior than women do (d = 0.33). In short, men on average report engaging in sex somewhat more than women do, and they also seem to want to engage in sex more than women do.

A recent study by David Schmitt and his colleagues (2003) provided strong evidence that men and women differ in their desire for multiple sexual partners. More than 16,000 men and women from 52 countries reported how many sexual partners they desired "in the next month" and "in the next 30 years." In all world regions and for both the short-term and the long-term, men desired significantly more sexual partners than women did (mean, d = 0.46). To control for a small number of men who desired extremely high numbers of sexual partners, Schmitt and his colleagues analyzed the percentage of men and women who desired more than one sexual partner in the next, month. Again, there were large and significant sex differences: Averaged across world regions, 25% of men, but only 5% of women desired more than one sexual partner over the coming month.

In another recent report based on data from more than 14,000 participants from 48 countries, David Schmitt (in press) asked men and women how restricted or unrestricted their sexual attitudes and behaviors were, Peopie with restricted attitudes tend to prefer committed and monogamous sexual relationships, and they view love and intimacy as prerequisites to sex. In contrast, people with unrestricted attitudes are more interested in sex with multiple partners and with little emotional commitment. Over all countries, the mean sex difference in committed versus uncommitted attitudes toward sex was quite large (d = 0.74), which implies that 77% of men have more uncommitted sexual attitudes and behaviors than the average woman does. However, there was also considerable variation in sex differences across countries (from a low of d = 0.30 and 0.39 in Latvia and Botswana, respectively, to a high of d = 1.24 in Morocco and Ukraine). Furthermore, the sizes of sex differences, across countries, were systematically related to the countries' cultural and economic characteristics. For example, countries with greater gender equality and more empowerment of women tended to have smaller sex differences in sexual commitment. Similarly, countries with more stressful economic environments (e.g., low economic development, high infant mortality rates) also tended to have smaller sex differences in attitudes toward sexual commitment (and more committed sexual attitudes in general). Despite the cultural variations in attitudes toward sexual commitment, however, in virtually all assessed countries, men were more uncommitted in their sexual attitudes than women were, and sex differences tended to be more than twice as large (in terms of effect sizes) as national differences in attitudes toward committed sex.

One striking aspect of sexuality has not been systematically studied in relation to sex differences, namely, the degree to which people are sexually attracted to men or to women. As far as I know, there have been no meta-analyses on this topic. I conducted a study in which I asked 285 college men and 429 college women to rate on 7-point scales how sexually attracted they were to men and to women (Lippa, 2000, Study 1). Not surprisingly, men reported on average being much more sexually attracted to women than women were (d = 3.52), and women reported on average being much more sexually attracted to men than men were (d = 3.99).

Differences of this magnitude imply that almost all men are more sexually attracted to women than the average woman is, and that almost all women are more sexually attracted to men than the average man is. The d value may be misleading in this case, however, because "sexual attraction to men" and "sexual attraction to women" are not continuous variables in the same sense that, say, smiling or aggression are. Most men are sexually attracted to women and are not sexually attracted to men. However, there is a minority of men—gay and bisexual men—who are sexually attracted to men. Similarly, there is a minority of women—lesbians and bisexual women—who are sexually attracted to women. Nonetheless, the d values illustrate the basic point that sexual attraction to men and sexual attraction to women show huge sex differences, on average.

Do men and women have different attitudes toward sex?

Psychologist Roy Baumeister (2000; see also Raumeister & Tice, 2001) has compiled evidence showing that women and men may show another fundamental difference in sexuality: Women's sexuality tends to be more flexible, variable, and responsive to social norms and settings, whereas men's sexuality seems more fixed, urgent, and unresponsive to social norms and settings. One piece of evidence that supports this proposition is that individual women report more variability over the course of their lives, both in sexual activity levels and in sexual orientation, than individual men do. In a sense, Baumeister argued, women's sexuality may be molded relatively more by nurture and men's sexuality more by nature. Drawing upon Baumeister's conclusions, University of California psychologist Let it ia Ann Peplau (2001) argued that, in general, women have a more partner- and relationship-centered orientation to sex, whereas men have a more recreational and body-centered orientation to sex. Stated simply, women are more sexually attracted to people and personalities, men to body parts and pleasurable sexual practices.

One reason environmental and cultural factors may have less of an impact on men's than on women's sexual behaviors may be that men, on average, have higher sex drives than women do, Roy Baumeister, Kathleen Catanese, and Kathleen Vohs (2001) presented a broad array of evidence supporting this conclusion. In questionnaire studies men report having higher sex drives, on average, than women do. Furthermore, numerous studies show that men, more than women, fantasize frequently about sex, desire frequent and varied sexual activities, are unwilling to postpone or forgo sex, take risks for sex, and endure costs to engage in sex. For example, men more than women risk social censure and criminal prosecution in the pursuit of sexual gratification (former President Bill Clinton provides a much publicized example); men are the main consumers of erotically explicit videos and magazines; and men are much more likely to purchase sex from prostitutes than women are.

Are There Sex Differences in Criteria for Mate Selection?

Do men and women look for the same characteristics in a mate? Alan Feingold (1992a) conducted a meta-analysis to answer this question. He found that women rated social class and ambitiousness to be more important in a mate than men did (d = 0.69 and 0.67, respectively). This implies that about 75% of women rate class and ambitiousness to be more important in a mate than the average man does. Women also rated character and intelligence to be more important in a mate than men did, but these differences were more modest (d = 0.35 and 0.30, respectively), There were still smaller sex differences in how important humor and personality were rated to be in a mate (d = 0.14 and 0.08, respectively), with women rating these more important than men.

Are there some traits that men rate to be more important in a mate than women do? Physical attractiveness is one (Feingold, 1990). In questionnaire studies, men rate a mate's physical attractiveness to be more important than women do (d = 0.54). And in studies that analyze the content of personal ads, men list attractiveness as a characteristic they are seeking in a romantic partner more than women do (d = 0.47). Men's greater preference for physical attractiveness in a mate proves to be quite consistent across different cultures (Buss & Schmitt, 1993).

Another trait more preferred in a mate by men than by women is youth. There is a strong norm in most cultures that, when there is an age difference in a marriage or couple relationship, the man should be older than the woman. Across cultures, as men age they increasing prefer women who are younger than themselves, whereas as women age, they seem to consistently prefer mates who are about their own age (Kenrick & Keefe, 1992).

Are There Sex Differences in Occupational Preferences and Interests?

According to vocational psychologist John Holland (1992), there are six main kinds of occupations: realistic, investigative, artistic, social, enterprising, and conventional (Holland, 1992; Fig. 1.5). Realistic occupations (e.g., mechanic, carpenter, plumber, and farmer) involve work with machines, tools, equipment, and farm animals. Investigative occupations (e.g., physicist, biologist, chemist) entail investigating physical, biological, behavioral, and cultural phenomena. Artistic occupations (e.g., painter, actor-actress, writer) involve manipulating physical, verbal, or human materials to create artistic products. Social occupations (e.g., minister/rabbi/priest, teacher, counselor) involve training, developing, counseling, managing, teaching, and directing other people. Enterprising occupations (e.g., sales associate, politician, stockbroker) involve manipulating other people to achieve organizational goals or to make money. Finally, Conventional occupations (accountant, file clerk, bookkeeper) require people to plan and operate business machines, process data, and keep records.

FIG. 1.5 Holland's six kinds of occupations.

Do men and women differ in their preferences for these six types of occupations? To answer this question, I conducted a meta-analysis of six studies that collected occupational preference data from more than 14,000 participants (Lippa, 2001b). My results showed that men prefer realistic occupations much more than women do (d = 1.06). This large difference implies that 86% of men prefer realistic occupations more than the average woman does. In contrast, women prefer social and artistic occupations more than men do (d = 0.62 and 0.63, respectively). Differences of this size imply that about 73% of women are more interested in social and artistic occupations than the average man is. Men are a bit more interested than women in investigative occupations (d = 0.32), and women are a bit more interested than men in enterprising occupations (d = 0.27). Men and women do not differ much in their preferences for conventional occupations (d = 0.06).

Research shows that there are two main dimensions that underlie people's preferences for Holland's six kinds of occupations: the people-things dimension and the ideas-data dimension (Lippa, 1998b; Prediger, 1982; Rounds, 1995; see Fig. 1.5). People-oriented occupations involve managing, thinking about, and interacting with other people. In contrast, thing-oriented occupations more often involve working with machines, equipment, and inanimate objects. (To get a sense of the difference between people who like people-oriented occupations versus those who like thing-oriented occupations, think about the differences between social workers and novelists, on the one hand, and engineers and computer scientists, on the other.) Idea-oriented occupations require creative thought and intellectual effort, whereas data-oriented occupations involve more routine and less intellectually demanding kinds of work (e.g., research scientists and fiction writers versus clerks and sales people).

Are there sex differences along the people-things and ideas-data dimensions of occupational preferences? The answer is respectively, yes and no. Men and women differ substantially on the people-things dimension (mean d = 1.29; Lippa, 1998b), but there is virtually no sex difference on the ideas-data dimension. A d value of 1.29 is quite large and implies that 90% of women are more people-oriented than the average man is, and conversely, that 90% of men are more thing-oriented than the average women is. Interestingly, these sex differences are already apparent in 2- to 4-year-old children (Goodenough, 1957).

The strong sex difference in people-orientation versus thing-orientation seems closely related to a recent distinction made by British psychologist Simon Baron-Cohen (2003), who proposed that women tend to be empathizers, whereas men tend to be systematizers. According to Baron-Cohen, higher female empathy shows itself in many ways: 1-day-old girls look more at faces, whereas 1-day-old boys look more at mobiles (Connellan, Baron-Cohen, Wheelwright, Ba'tki, & Ahluwalia, 2001). Girls are more interested than boys in social toys (e.g., dolls), and they like to nurture people or people surrogates, such as pets. Girls often have a more developed theory of mind than boys do; girls are better able to infer others' thoughts, feelings, motives, and intentions. Similarly, women are more focused on interpersonal relationships than men are; they give and seek social support more, and they are more collaborative and reciprocating in their conversations.

Males higher tendency to systematize similarly shows itself in many ways. Boys are more interested than girls in mechanical and construction toys; they like things: cars, trucks, blocks, guns, and swords. Similarly, men are more drawn than women to occupations that focus on machines, mechanisms, and deterministic systems, occupations such as carpentry, computer science, car repair, physics, and engineering. Males are more interested than females in hobbies and activities that involve collecting and organizing things: stamp collecting, compact disc collecting, baseball card collecting, trainspotting, birdwatching, and sports record-keeping. According to Baron-Cohen, the most extreme systematizers in the world are autistic individuals, who can be brilliant in specific domains such as computation and visual memory but who are inevitably inept at understanding other people and negotiating human relationships. Autism is much more common in males than females. This is no accident, according to Baron-Cohen, who argues that some kinds of autism result from having "an extreme male brain."

Are There Sex Differences in Cognitive Abilities?

Many psychologists conclude that there are no meaningful sex differences in general intelligence (Halpern, 1992, 1997, 2000; Jensen, 1998). However, sex differences are sometimes found for specific kinds of mental abilities. On average men perform somewhat better than women on tests of math ability (d = 0.43, based on Rosenthal and Rubin's 1982 and Becker and Hedges' 1984 reanalyses of data from Hyde, 1981). In contrast, women perform somewhat better than men do on tests of verbal ability d = 0.11 according to Hyde and Linn's (1988) estimate and d = 0.24 according to Hyde's (1981) estimate.

Although the female advantage in general verbal ability appears to be small, there are some specific verbal tasks, such as spelling, verbal fluency, and verbal composition, in which women show a more substantial advantage over men (Haipern, 1992, 1997, 2000).

On average, men score higher than women on many kinds of visual-spatial tests (d = 0.45; Linn & Peterson, 1986; see also Voyer, Voyer, & Bryden, 1995), and this difference is particularly strong for tests of mental rotation (d = 0.73 in Linn and Petersen, 1986; d = 0.56 for all ages and 0.66 for participants over 18 years of age in Voyer, Voyer, and Bryden's review). A d value of 0.73 implies that 77% of men score higher on mental rotation tests than the average woman does. Mental rotation tests assess how well a person can mentally turn around three-dimensional representations of objects. Men also do better than women on water-level tests, which ask participants to estimate the surface created by water in containers that are turned to various orientations (d = 0.42 in Voyer, Voyer, and Bryden's meta-analysis), women, however, outperform men on tests of spatial location memory, which ask participants to remember, for example, where various objects are located throughout a room after brief observation (Eals & Silverman, 1994).

Finally, recent evidence suggests that boys and men tend to have more general knowledge than girls and women do. Analyzing data from six large samples used to create norms for the Wechsler Intelligence Test, Northern Ireland psychologists Richard Lynn, Paul Irwing, and Thomas Cammock (2001) found that men consistently scored higher than women on the Wechsler information subtest (mean, d = 0.36). They also developed a 182-item test of general knowledge, and when they gave their test to a group of Northern Ireland college students, they found that men generally possessed more knowledge than women did (overall, d = 0.51, which implies that 69% of the men had more general knowledge than the average woman did). For some knowledge subdomains, the male advantage was quite large: d = 0.82 for current affairs knowledge (which comprised knowledge of politics, finance, history, and geography), d = 0.75 for knowledge of physical health and recreation (biology, games, and sports), and d = 0.58 for science knowledge (general science and history of science). Two knowledge domains that showed a female advantage were medicine (d = 0.32) and food and cooking (d = 0.48).

Similar findings come from a study by Phillip Ackerman, Kristy Bowen, Margaret Beier, and Ruth Kanfer (2001), which assessed various kinds of knowledge in more than 300 students at selective Georgia universities. Men showed higher overall knowledge than women did (d = 0.68). Ackerman and his colleagues also found significant sex differences in subdomains of knowledge. For example, men exceeded women in their knowledge of technology (d = 1.04), electronics (d = 0.98), physics (d = 0.72), geography (d = 0.66), and history (d = 0.56). Knowledge domains showing no significant sex differences included literature, business, art, and psychology. In this study, no knowledge domains showed a female advantage. Other recent studies pointed to cross-cultural consistency in sex differences in knowledge. For example Evans, Schweingru ber, and Stevenson (2002) found that boys possessed moderately higher general knowledge than girls in samples of 11th grade students in the United States, Taiwan, and Japan; Lynn, Wilberg, and Margraf-Stiksrud (in press) reported similar findings among German high school students.

Are There Sex Differences in Physical Abilities?

Several meta-analyses have summarized sex differences in physical abilities (Eaton & Enns. 1986; Thomas & French, 1985). On average, men show higher activity levels than women do (d = 0.49). However, women show better fine eye-hand coordination than men do (d = 0.21), and their joints and limbs are more flexible than men's (d = 0.29). Men can throw objects faster, farther, and more accurately than women can (d = 2.18, 1.98, and 0.96, respectively). The sex difference in throw velocity indicates that some 99% of men can throw faster than the average woman can, and the difference for throw distance indicates that about 98% of men can throw farther than the average woman can. Seventy-seven percent of men can throw objects more accurately than the average women can. On average, men's grip strength exceeds that of women (d = 0.66). Men also perform better than women on tests of sit-ups, short run speeds, and long jumps (d = 0.64, 0.63, and 0.54, respectively). A d value of around 0.6 implies that 73% of men perform better than the average woman does. Many of these findings reflect the fact that men have greater upper body strength than women do.

Are Men More Variable Than Women on Some Traits?

Virtually all the evidence presented so far has focused on mean differences between men and women, differences in personality, risk-taking, aggression, and so on. However, men's and women's trait distributions are characterized by their spread (what statisticians term the standard deviation or variance of a distribution) as well as by their means. Does the spread of male and female distributions differ for some traits? Who is more variable, males or females? Analyses of data from cognitive ability tests have provided evidence that men are more variable than women, particularly in mathematical and visual-spatial ability (De Lisi & McGillicuddy-De Lisi, 2002; Feingold, 1992b; Hedges & Nowell, 1995). A group of Scottish social scientists recently presented compelling new evidence that boys are more variable than girls in overall 1Q (Deary, Thorpe, Wilson, Starr, & Whalley, 2003). Analyzing intelligence test scores collected from virtually everyone born in Scotland in 1921, these researchers found virtually no difference in the mean IQ scores of 11-year-old boys and girls (who were assessed in 1932). However, boys' IQ scores were significantly more variable than girls', with the net result that there were more boys than girls with both subnormal and gifted IQs.

Recent meta-analyses by John Archer and Mani Mehdikhani (2003) suggested that greater male variability extends to other traits as well, including physical aggressiveness and the degree to which men and women seek various characteristics in mates, such as money, ambition, chastity, and being older or younger than oneself. If men are more variable than women on certain traits, then this could have important real-life consequences. Greater male variability in aggressiveness, for example, in combination with higher mean male levels or aggressiveness, could lead to very large sex differences at the highest levels of aggressiveness. Consistent with this hypothesis, a study of more than 13,000 same-sex murders found that a staggering 97% consisted of men killing men, but only 3% consisted of women killing women (Daly & Wilson, 1990). Thus homicidal aggression—a decidedly extreme form of aggression—is much more characteristic of men than women. Similarly, greater male variability in math ability, in combination with moderately higher mean male levels of math ability, could lead to very large sex differences at the highest levels of math ability. This may help explain why 13 times as many boys as girls score above 700 on the math section of the SAT (Scholastic Aptitude Test; Benbow & Stanley, 1980, 1983).