Invited Lecture - 2nd to 4th digit ratio: a correlate of prenatal testostedrone and sex dependent traits.
John Manning, Department of Psychology, University of Central Lancashire, UK.

Sex dependent traits such as male pattern baldness are often influenced by genes and by sex steroids such as testosterone (T). The production ofT begins in the male foetus at around week 8 and peaks at about week 13. Thereafter it reduces, shows a transitory increase after birth, and is then at low levels until puberty. It is thought that foetal T has major effects on the organisation of many tissues (e.g. the skin) and organ systems (e.g. the brain), and the activational effects of adult T may be determined by early exposure to prenatal T. Here I introduce a probable proxy for prenatal T, the relative lengths of the 2nd (index) and 4th (ring) fingers or 2D:4D. There is evidence that 2D:4D is highly conserved in that it is sexually dimorphic in mammals, birds and reptiles. In humans 2D:4D ratio is dimorphic such that males have lower mean 2D:4D than females, the sex difference appears in foetuses as young as 9 weeks, and although there are ethnic differences in 2D:4D the sex dimorphism is universal in humans. Low 2D:4D, particularly low right hand 2D:4D, is related to high in utero T and right -left hand 2D:4D may be positively associated with sensitivity to T as indicated by the structure of the androgen receptor gene. I discuss how the correlates of2D:4D range from sex dependent behaviours such as handedness, strength and running speed, to measures of sperm function and family size, and to predispositions to diseases such as toxoplasmosis, eczema, and myocardial infarction.