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Heredity (2002) 89, 403
2002 Nature Publishing Group All rights reserved 0018-067X/02 $25.00
www.nature.com/hdy
Book review
Digit Ratio: A Pointer to Fertility,
Behavior, and Health
John T Manning
Rutgers University Press, New Jersey. 2002; 173 pp. $27.00,
paperback. ISBN 0-8135-3030-X
Heredity (2002) 89, 403. doi:10.1038/sj.hdy.6800151
The main message of this book is simple and John Man-
ning does an excellent job of delivering it clearly. The
ratio of second and fourth (finger) digit lengths (2D:4D)
appears to be affected by foetal testosterone exposure,
may be fixed before birth, and so may be a reliable indi-
cator of early prenatal conditions and subsequent
health factors.
Through sequential chapters, Manning relates 2D:4D to
various evolutionary concepts such as reproductive suc-
cess, individual performance, heritability, and mate
choice; to social factors such as aggression, sexual prefer-
ences, and individual status; through to more applied
medical studies of health and illness. Clearly, some of
these relationships are better supported than others, and
Manning presents the weakest correlations with a few
well-placed caveats.
In general, the book builds from relatively basic claims
to declarations that many readers will construe as ‘over-
interpretation’. If we take Manning’s conclusions literally,
you could predict a team’s performance at the next World
Cup by the digit ratio of team members (p 139) – the
lower 2D:4D the better their chances. This means we
should all head to Ladbrokes and bet on rank outsiders
Jamaica – they have the lowest digit ratio of the cultures
sampled so far (p 19), and yet will offer quite stagger-
ing odds!
The book is aimed as an introductory text into this field
and, as such, presents data that has been analysed with
varying statistical rigour. Manning also adopts a fairly
uncritical tone throughout. It is fair to say that several of
the correlations presented have marginal significance,
and r
2
values and effect sizes are largely not reported.
Many bivariate plots resemble shotgun blasts, and much
of the data are not published. Maybe this will change as
the field grows. Or perhaps we should expect weak corre-
lations when analysing such complex, multidimensional
traits (such as human sexual orientation)?
I was a little surprised at the apparent holes in the data.
It is unclear to what extent 2D:4D is affected by foetal
testosterone, and how much of the variance in the digit
ratio of adults is explained by uterine conditions. Man-
ning reports a lack of longitudinal data on relative
changes in finger lengths with age. Yet several of his own
datasets point to digit ratio differences among age
cohorts (eg, pp 87 and 91). Also, he has previously
reported significant within-individual changes in finger
dimensions over short time periods (Manning et al, 1996;
Scutt and Manning, 1996; Manning et al, 2002). Person-
ally, I am not sure how to reconcile these discrepancies
and it would have been useful for Manning to address
this head-on.
Readers will undoubtedly adopt more welcoming or
more damning views of the book’s thesis. Clearly this is
a field of interest to many health care professionals and
yet some fundamentally important questions remain
unexplored. Manning communicates this well. Every
student entering graduate school in a related field should
be licking their chops! There is plenty to achieve here.
With this in mind, I hope that researchers working in this
field steer clear of wholly adaptive scenarios and adopt
quantitative rigor in interpreting the strengths and weak-
nesses of their data. The field of digit ratio research, and
any medical advances that accrue, will be better for it.
The book is designed to open people’s eyes to possi-
bilities rather than make definitive statements. There is
just too little data at present. Manning’s style is jovial,
easy to read, and will make digit ratio research interest-
ing and accessible to many. This is a great book for stu-
dents to read, but may frustrate many seasoned evol-
utionary biologists. If it is read with the appropriate
caveats in mind, it will be a good basis for seminars and
discussion classes.
References
Manning JT, Gage AR, Driver MJ, Scutt D, Fraser WD (2002).
Short-term changes in asymmetry and hormones in men. Evol
Human Behav 23: 95–102.
Manning JT, Scutt D, Whitehouse GH, Leinster SJ, Walton JM
(1996). Asymmetry and the menstrual cycle in women. Ethol
Sociobiol 17: 129–143.
Scutt D, Manning JT (1996). Symmetry and ovulation in women.
Human Reprod 11: 2477–2480.
JP Swaddle
Institute for Integrative Bird Behavior Studies
Biology Department
College of William & Mary
Williamsburg VA 23187-8795, USA
E-mail: jpswad@wm.edu