Digit Ratio: A Pointer to Fertility, Behavior, and Health (The Rutgers Series in Human Evolution) [ペーパーバック]

This book by John T. Manning, who is apparently the leading authority on digit ratio and its biological/hormonal causes.

To summarize: this book posits and goes about proving how prenatal testosterone levels affect the masculinazation of the fetus, resulting in a low 2d:4d digit ratio. This ratio is the ratio of finger length of the index finger vis-a-vis the ring finger on the same hand. Masculine ratios are under 1.0, meaning that the index finger is shorter than the ring finger. Feminine ratios are generally 1:1 or the index finger is slightly longer than the ring finger. Manning also goes on to determine how this ratio may be an indicator of adult masculine traits, such as athletic ability, musical ability, and physical aggressiveness and assertiveness.

What this book does right:

1. It's more of a textbook than casual reading. It presents copious data and is immaculately presented and organized in easy to read fashion. It is easy to refer back to over and over again, as it should be with good reference material;

2. As such, this book will dispel any disbelief in what is considered a mildly controversial topic in mainstream news. People are still uncomfortable with the fact that something as obvious as the finger length on their hands can tell a lot about them, even to total strangers;

3. Despite the copious anecdotal evidence and hard data, Manning does exercise due caution in jumping to conclusions about where this area of study may lead;

4. The prose is very succinct and to the point. Though only around 170 pages, it is packed with information;

This book only gets four stars because of some minor quibbles. First, it is not a book that will entertain the casual science reader. Those who will get the most benefit are those who are actually in a field of biological study that deals specifically with areas like prenatal hormonal conditions, sex differentiation of brain patterns, etc. Second, Manning may be overly conservative in conjecturing where this field of study may lead. Granted, he is justified in doing so, given the potentially controversial and ethically questionable course it may take; however, I felt this book leaves almost too much to further study, given the fact that scientists have known about the 2d:4d ratio and its biological roots for decades. At this stage, this book could have been written as a roadmap for further study rather than being mainly a recap for what has already been done.

Still, it is the only book of its kind that I know of, and it does very ample justice to a fascinating and obscure topic that literally affects all of us.

Bottom line: a must-read for people interested in the biological origins of homosexuality.

Many deep secrets are right at our fingertips! Literally! But get ready to wade through plenty of dry prose and lots of graphs to find out.

This book explains what scientists have learned about neurobiology and genetics from studying finger lengths. You can participate in this ongoing research on the web at the BBC. Preliminary results are in from controlled studies, and they give the clearest picture so far of the origins of homosexuality in men, which appears to correlate with the fetus and the fetal brain being marinated in high concentrations of testosterone at a very specific point in early pregnancy--a point that also determines the ratio of your index finger to your middle finger on your dominant hand. The author can only speculate why the mother's body reacts to the viable embryo with this particular hormonal response, and it is noted (as has been noted elsewhere in popular science) that first-born sons are less likely to be gay than subsequent male offspring of the same mother. More data is required to correlate digit ratio with female homosexuality, and the book goes into several other useful applications of the findings for medical diagnostics.