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Watch an exclusive video about John Manning and his theories

YOUR fingers are colossal tattletales. According to evolutionary psychologist , who has spent years researching the secrets they betray, your digits reveal all sorts of things, from your sexuality to whether you are socially disadvantaged or likely to suffer a heart attack. They even, as the legend goes, give away the size of a man鈥檚 penis.

The key, Manning says in The Finger Book, is the 2:4 ratio: that is, the relative lengths of your index finger (the second digit) and your ring finger (the fourth). Men tend to have relatively long ring fingers, whereas the two are often the same length in women. The 2:4 ratio for white males in north-west England, for instance, is 0.98; for white females, 1.00.

Relative finger lengths appear to be fixed in the womb by about the 13th week, probably a function of testosterone exposure in the uterus, Manning says. If that鈥檚 true, relative finger length is like a fossil record of early prenatal life.

His findings are intriguing. People with long ring fingers, for instance, tend to end up in maths, physics and engineering. They run well, draw well, are good at perceiving shapes and produce lots of high-quality sperm. People with autism also tend to have low 2:4 ratios, as do musicians.

Studying members of a British symphony orchestra, Manning found that they had lower, more 鈥渕asculine鈥� ratios than the general public. The higher ranking the musician, the lower their ratio tended to be.

Manning thinks that just as high prenatal testosterone levels made the ring fingers long, they also affected brain development. Musical ability may be a signal of male fitness: 鈥淚n short,鈥� he writes, 鈥渕en who make lots of good music make lots of good sperm.鈥�

The Finger Book is wonderfully provocative and a good read. But it鈥檚 not clear why these particular fingers are so important. Where is the hard evidence that they are preferentially affected by sex hormones? Then there鈥檚 the fact that finger-length ratios vary widely by ethnicity. Jamaican women, for instance, have an average 2:4 ratio of 0.95 鈥� more 鈥渕asculine鈥� than British white men. Did they really get more testosterone as fetuses? That question was raised but never quite answered. Like other provocative ideas in this book, it needs explaining before the finger theory holds up.