Hormones and Maternal Behavior
The Case for Nature
Estrogen and other hormones (most notably, progesterone, which prepares females' bodies for pregnancy; prolactin, which triggers uterine contractions during labor and stimulates the milk let down reflex in nursing mothers; and oxytocin, which is implicated in attachment, pair-bonding, and romantic love) likely play a role in fostering maternal behavior.
Animal research (e.g., on rats; see Bridges, 1990; Fleming & Li, 2002) shows that females who have experienced pregnancy and birth seem primed to display maternal behaviors; that is, they respond to helpless infants more readily and nurture newborns more than males and inexperienced females do. Such priming probably results, in part, from the effects of hormones. Virgin female rats injected with blood from new mother rats become more maternal, probably because of hormonal transfers (Terkel & Rosenblatt, 1968). And virgin female rats given estrogen and progesterone are more likely to respond to the ultrasonic distress calls of rat pups than nontreated females are (Farrell & Alberts, 2002). It probably is no accident that most mammalian females eat the placenta after giving birth (Kristal, 1991). They not only receive painkilling opiates, electrolytes, and a nutritious pick-me-up by doing so but also likely ingest a potent dose of hormones, which helps to trigger their attachment to offspring and initiate maternal behaviors.
Although hormones play a role in maternal behavior, it is important to note that their influences are not fixed or rigid. Females vary considerably in their maternal instincts, depending on circumstances and previous experience. Virgin rodents can often be indifferent and even hostile toward pups. Evolutionary anthropologist Sarah Hrdy (1999) observed, "A virgin female rat... will either ignore or devour a pup she happens upon. But if she is repeatedly exposed to pups, this inexperienced 'au pair from hell' becomes quite nurturing—without undergoing the hormone changes specific to pregnancy. When experimenters place pups in her cage again and again, eventually she stops killing and begins to care for them (p. 151)."
The same is true for male rats. With repeated exposure and experience, they too will learn to retrieve pups and gather them into a nest (Bridges, Zarrow, & Denenberg, 1973). Furthermore, hormonal manipulations can make males more broody. For example, males castrated early in life, which removes the influence of androgens, show increased levels of maternal behaviors later in life. When adult males are given pregnancy-related hormones such as estrogen, progesterone, and prolactin, they also show increased levels of maternal behaviors (Bridges, Zarrow, & Denenberg, 1973; Rosenberg & Herrenkolh, 1976). Such findings are important because they suggest that the neural bases for maternal behavior are present in male mammals' brains too, even if such behaviors are more likely to show themselves after hormonal priming or in favorable environments. Charles Darwin (1836-1844) noted the possibility of male maternal behavior when he wrote in his notebook: "A capon [a castrated rooster] will sit upon eggs, as well as, and often better than a female.... [T]his is lull of interest; for (there are) latent instincts even in brain of male (p. 380, passage 154)."
Although in most mammalian species, mothers take care of their offspring much more than fathers do, there are some exceptions. Among wolves and South American tite monkeys, for example, fathers "mother" more than mothers do (Mendoza & Mason, 1986; Whitten, 1987; Yogman, 1990), Conditions that foster male nurturing of young include monogamy and male certainty of paternity. Research on humans shows that in most human societies, including modern industrial ones, women take care of children more than men do (Parke, 2002; Pleck & Masciadrelli, 2004). Questionnaire studies suggest one explanation: Women are simply more interested in babies than men are (Berman, 1980). Do hormones play a role in these differences, or does greater female mothering result from gender roles, social expectations, and the fact that women bear children, lactate, and therefore necessarily bond more with their offspring, early on, than fathers do? Hormones likely play a role. However, as always, it is hard to disentangle the effects of nature and nurture.
Female monkeys and primates often show intense interest in being allomothers, that is, substitute mothers who babysit others' infants (Nicolson, 1987). Displaying strikingly parallel traits, girls are generally much more interested than boys in role-played mothering, for example, in playing with babydolis (see Chapter 1). Of course, this may result from social learning as well as from the effects of hormones. Arguing in favor of a role for sex hormones, however, is a recent study that showed that vervet monkeys (like human boys and girls) show sex differences in toy preferences: males play more with cars and balls, and females play more with dolls and pots (Alexander & Hines, 2002). This suggests that female primates' tendency to be more attracted than males to infants and infant surrogates (like dolls) results, at least in part, from the early organizational effects of sex hormones. Male and female vervets are unlikely to be following the dictates of culture or gender stereotypes.
Additional evidence for hormonal effects on maternal behaviors comes from studies on CAH girls and women, who are exposed to high prenatal levels of androgens because of an enzymatic defect. CAH girls are less interested in feminine play and in dolls than non-CAH girls are (Berenbaum &Hines, 1992; Leveroni & Berenbaum, 1998). Furthermore, CAH girls and women report that they are less interested in babies and in having children than control groups are (Dittmann et al., 1990). These findings provide suggestive evidence that early hormonal factors in girls can affect their later levels of maternal interest. Similar hints come from a study by sociologist Richard Udry (2000), who found a correlation between females' exposure to testosterone in fetal life and their adult levels of masculinity and femininity, including their interest in rearing children.