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DIGIT RATIO (2D:4D): A PRENATAL MARKER FOR ANDROGEN RELATED TRAITS.
J.T.Manning
Population Biology Research Group, School of Biological Sciences,
PO Box 147, University of Liverpool, Liverpool L69 3BX, UK jtmanning@uclan.ac.uk
It has been known for some 100 years that digit ratio (2D:4D), i.e.
the relative lengths of the 2nd (index finger) and 4th (ring finger),
shows a sex difference such that males tend to have longer 4th fingers
relative to their 2nd than do females. The sex difference is quite
small and attracted little attention. However, in 1998 it was suggested
that 2D:4D was related to prenatal levels of testosterone and oestrogen.
Since that date some 170 papers have been published on 2D:4D. If
the link with prenatal sex steroids is correct it means that 2D:4D
is a sort of living fossil in the fingers. As such it bears testimony
to our exposure to foetal androgens, and the implications for health
and disease such exposure entails.
In this talk I will present evidence that the sexual dimorphism in
2D:4D is an ancient trait found in four-limbed vertebrates including
reptiles (lizards), birds and mammals (e.g. mice, rats, baboons).
In humans low values of 2D:4D (high prenatal testosterone) are characteristic
of males and high values of 2D:4D characteristic of females. However,
there is much overlap between the sexes. The sex difference in 2D:4D
is found in the foetus as early as the 8th week, children and adults
and is quite stable with growth in childhood and changes little
at puberty. The dimorphism is universal within all human groups,
but 2D:4D varies considerably between macro-ethnic groups such that
Caucasians have high 2D:4D (low prenatal testosterone) compared
to East-Asians and Blacks. The differences between some ethnic groups
are as large as differences between the sexes. Within ethnic groups
there appears to be population differences in 2D:4D.
The evidence that 2D:4D is influenced by prenatal testosterone is
less convincing than the morphological evidence of sex differences.
Such data as we have will be reviewed from studies of children with
congenital adrenal hyperplasia, opposite-sex twins, amniocentesis
samples, and the structure of the androgen receptor gene.
There have been many studies which have linked 2D:4D with sexual
behaviour and fertility, consumption of drugs, occupational preferences,
function of the immune system, various cancers, physical fitness
and heart disease, and mental health. I review the evidence that
low 2D:4D (high prenatal testosterone) is associated with large
families in men but small families in women; polycystic ovary syndrome;
male heterosexuality but not female homosexuality; preferences for
'male-type' occupations in both men and women and membership of
'male-type' occupations in women; left handedness; high consumption
of alcohol but low consumption of prescribed drugs; protection against
breast and ovarian cancer; fast running speeds over various distances
but particularly in endurance events, physical fitness, punishing
training schedules and performance in various sports such as soccer;
protection against myocardial infarction and low fibrinogen levels;
a predisposition towards high parasite loads, protection against
various psychopathologies such as neuroticism and schizophrenia
but a predisposition towards other psychopathologies and developmental
disorders including autism, Asperger's syndrome, hyperactivity and
poor social cognition.
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