It has been long established that there are considerable psychological differences between males and females that have been demonstrated in a range of behavioural patterns such as aggression, sexual orientation, emotional responses and, in the case of children, toy/ activity preferences (Hines, Johnston, Golombok et al, 2002). The relative impact of nature and nurture factors continues to be debated within biomedical sciences, but it would appear that hormonal factors during early development may have a critical role in understanding behavioural differences between the sexes. This paper will examine recent evidence on the relationship between hormones and sex differences, drawing on research studies that have included normal population samples, and also clinical population samples such as children born with inter-sex conditions. It will outline the methodological difficulties associated with research in this area.
Recent research has shown that many sex differences can be attributed to differences in hormonal and stress responses during the pre-natal phase. However, the actual relationship between hormones and sex differences in humans has largely been based on experimental animal-based models, partly due to ethical concerns associated with biomedical research on young human infants. Nevertheless, this is vital research since newborn male infants are at increased risk of mortality and morbidity (Elsman, Steen and Hellstrom-Westas, 2004) and early hormonal environment may have a critical role in understanding sex-related health differences. One hypothesis on the interaction of hormones and subsequent behavioural differences between the sexes points to the role of prenatal stress causing hormonal changes in the developing foetus (Hines et al, 2002). In animal experiments, pregnant rats that are stressed through adverse conditions produce litters than include male rats with less masculine behaviour and reduced neural development in cortical areas responsible for sexual behaviour (Ward, 1984). Furthermore, male rats that are castrated at birth demonstrate less masculine behaviour, and more female behaviour, as adults (Goy and McEwen, 1980). The impact on prenatal stress on female rodents would appear to affect fertility and fecundity, and promote male-type sexual behaviours (Goy and McEwen, 1980).
Similar experimental work cannot be replicated in human infants for ethical reasons. However, research studies would appear to show that adult sexual preference is not unduly affected by stress-related factors during pregnancy, although research findings have been mixed. While Ellis, Ames, Peckham and Burke (1988) report a marginally significant effect of pre-natal stress during the second trimester of pregnancy, on the heterosexual/ homosexual preferences of adults, another study has found no such relationship (Schmidt and Clement, 1990). However, such research requires careful interpretation in view of the methodological limitations of retrospective reporting about stressful pregnancy events that may have occurred very many years prior to interview and maybe vulnerable to social responding. Furthermore, such research that implies homosexuality has some underlying pathological basis may not be ethically appropriate at the present time.
While early hormonal environment may contribute to sex differences, it cannot account for all differences between males and females. Hines et al (2002) have considered the role of prenatal stress in childhood gender role behaviour using large-scale, prospective data on child development. Parents completed a range of self-report measures regarding pregnancy and the gendered behaviour of their infants at 42 months of age. This study found that maternal self reported personal stress during pregnancy had a small association to the gender-related behaviours of female infants, and no association to the gender-related behaviours of male infants. Hines et al (2002) speculate that these results conflict with animal experiments because there are differences in gestational period between animals and humans, particularly in the phase of pregnancy associated with sexual differentiation. It could be that the length of the human pre-natal period may allow for compensatory strategies by the developing foetus against an adverse hormonal environment. Hines et al (2002) observed that a number of factors contributed to gender role behaviour of small children. The strongest predictive factor for gender-related behaviour of the infants was the sex of siblings in the family. In particular, children with older brothers demonstrate more masculine behaviour, whereas older sisters are associated with feminised behaviour.
Furthermore, children from a two-parent traditional family where parents are engaged in traditional gender-specific roles in terms of domestic responsibilities in the home demonstrated more gender-typical behavioural patterns. Alcohol/ smoking status of the mother and maternal education also had a smaller role. Therefore, this study suggests a small role for early hormonal environment and a much larger role for post-natal socialisation in understanding behavioural differences between children of different sexes. However, although this was a large scale study with nearly 14000 families included, it was dependent on retrospective self report of pregnancy related stress as opposed to biochemical measurement of stress during pregnancy which may have been a more objective approach to studying the interaction of hormones, stress and behavioural outcomes. For example, Van De Beek, Thijssen, Cohen-Kettenis et al (2004) have overcome the methodological difficulties associated with retrospective reporting through their study of the influence of prenatal sex hormones on subsequent human development with 156 expectant mothers. Their study measured hormones in the amniotic fluid in the 2nd and 3rd trimester of pregnancy. As expected, there were significant differences between the male and female foetus’ in terms of testosterone, estrogen and androgen obtained. Furthermore, Cohen-Bendahan, Van De Beek and Berenbaum (2005) have summarised the existing evidence from normal and clinical populations on the impact of prenatal sex hormones on child and adult subsequent behaviour. The physical and behavioural effects of prenatal stress would appear to be related to hormonal changes in the pituitary-adrenal-hypothalmic centre. Their comprehensive review shows that temperament of babies, cognitive functions such as spatial ability, physical strength in childhood and adulthood, physical characteristics such as finger size, sexual orientation as well as aggression and attention span in infancy are influenced by early hormonal environment. As Cohen-Bendahan et al (2005) suggests, “there is increasing convergence of evidence across methods showing the masculinising effects of prenatal androgens, especially at high doses of androgens…it seems likely that androgens are responsible for some of the differences between the sexes in these traits, although it is not as clear how much they contribute to variations within males and females” (p 377). One way to understand the relative impact of hormonal environment on the individual is to consider the outcome of cases where a child is born with an inter-sex condition. For example, women born with congenital adrenal hyperplasia differ from unaffected females in a number of domains including activity levels, sexuality and higher spatial ability than other women (Cohen-Benadaham et al, 2005). This type of research underlies the important role of early hormonal environment in the development of abnormalities, and the subsequent behavioural outcomes of such disorders.
In conclusion, research shows that pre-natal hormonal environment has an important role in the subsequent development of individuals. However, it does not account for all observed sex differences, and research has also shown an important role for early gender socialisation in the behavioural outcomes of individuals during childhood and adulthood. The relative impact of sex hormones and social factors on childhood development are not fully understood in the present research evidence and there are many methodological weaknesses in existing research studies. Therefore, further stratified, prospective research including hormonal sampling from the mother and baby, with long term follow up in the post-natal period are required to understand the relative importance of hormones, as opposed to gender socialisation, in the development of sex differences. It is only through such work that the relative contribution of sex hormones to behaviour can be understood.
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