The role of chromosomes and hormones
Biological sex is determined by the sex chromosomes X and Y — XX for females and XY for males — with sex chromosomes containing genetic material that controls development as a male or female, a process assisted by sex hormones, such as testosterone, oestrogen and oxytocin. Testosterone stimulates development of male sexual characteristics and is associated with masculinisation of the brain, for example the development of spatial skills, and male-type behaviours, such as competitiveness. Oestrogen promotes the development of female sexual characteristics and assists in the feminisation of the brain, as well as female-type behaviours, for instance co-operation. Oxytocin is involved in reproductive behaviours — it controls contractions during childbirth, stimulates milk production during breastfeeding and is involved in pair bonding.
Klinefelter’s syndrome (KS) is a male genetic condition involving an extra X chromosome. KS occurs during meiosis, where egg or sperm cells produce an extra copy of the X chromosome. Males produce insufficient levels of testosterone to allow physical male sexual characteristics to fully develop. Psychological effects include poor language skills and a passive temperament. Turner’s syndrome (TS) is a female genetic condition, occurring at conception, which involves having an extra Y chromosome. Non-functioning ovaries result in underdevelopment of female sexual characteristics, for example fertility. There are physical effects, such as down-slanting eyes, as well as potential abnormalities of the heart and kidneys.
Fig 10.2 The sex chromosomes X and Y determine human biological sex
Stochholm et al. (2012) assessed whether males with KS had a greater incidence of criminality by investigating criminal patterns in 1,005 Danish men with standard KS and an XYY version. It was found that men with KS had higher conviction rates for sexual abuse, burglary and arson and lower conviction rates for traffic and drug offences than non-KS controls and these findings were even greater for those with the XYY version of KS. However, when adjustments were made for socioeconomic variables (such as level of education, fatherhood, cohabiting with a partner, etc.), conviction levels were similar to the controls (apart from sexual abuse and arson). This suggests that KS is associated with increased criminality, but through poor socioeconomic conditions endured by sufferers rather than the condition itself.
• Money & Ehrhardt (1972) reported on girls whose mothers took drugs containing testosterone during pregnancy. The girls showed male-type behaviours, like playing energetic sports, and an absence of female-type behaviours, such as playing with dolls, which suggests that testosterone influences gender behaviour.
• Alonso & Rosenfield (2002) found oestrogen necessary for the normal development of body tissues, such as the neuroendocrine—gonadal axis, associated with puberty in males and females. This supports the idea that oestrogen is associated with the transformation into being sexually active in females and males too.
• White-Traut et al. (2009) measured oxytocin levels in saliva produced by females before, during and after breastfeeding. Oxytocin levels were highest just before feeding, decreased at initiation of feeding and rose again 30 minutes after feeding, illustrating oxytocin’s role in promoting breastfeeding.
• Price et al. (1986) performed a longitudinal study for 17 years of 156 females with TS, finding 9 per cent died compared with 3 per cent in matched non-TS females. Most died of cardiovascular and circulatory conditions, which suggests TS sufferers have a short lifespan due to organ abnormalities.
Testosterone is not exclusively a male hormone or oestrogen and oxytocin exclusively female hormones — research shows all three exert influences on both males and females.
Isotocin is a type of oxytocin found in non-mammals, which causes females to respond to male mating songs. It acts primarily on auditory stimuli in females, so may explain why women are attracted to male ’crooning’ singing voices.
As both KS and TS are biological conditions caused by atypical chromosome patterns, research into them is justifiable on the basis that it could bring about effective gene therapies to correct the conditions.
It is simplistic to regard single hormones as having exclusive effects on sex and gender. Hormones are part of biological mechanisms exerting multi-faceted and complex actions upon the body and behaviour, for example the interaction of sex chromosomes, the SRY gene and gonadal hormones.
Much research into the role of hormones is conducted on animals, presenting problems in generalising findings to humans.
Both KS and TS are diagnosed prenatally on cytogenetic analysis of a foetus. However, ethically this may be socially sensitive, as such a diagnosis could lead prospective parents to demand an abortion.
Oxytocin is given during childbirth to reduce bleeding, decreasing death rates in both mothers and newborn babies. Testosterone is given to men with problems in achieving and sustaining erections, while oestrogen protects post-menopausal women against osteoporosis.