Men and women blink differently when startled. That simple and well-established observation has led Qazi Rahman of the University of East London, in England, and his colleagues to evidence supporting the idea that homosexuality is a characteristic which people are born with, rather than one they acquire as they grow up. The team's research, just published in Behavioral Neuroscience, shows that lesbians blink like heterosexual men. That, in turn, suggests that the part of their brain that controls this reflex has been masculinised in the womb.

Anyone who is startled by an unexpected noise tends to blink. If, however, the startling noise is preceded by a quieter sound, this blink is not so vigorous as it would otherwise have been. It is this lack of vigour which differs between the sexes. Men blink less vigorously than women when primed in this way.

Qazi Rahman and colleagues' paper is published in Behavioural Neuroscience.

Given such a clear and simple distinction, testing the responses of homosexuals to noise seemed an obvious experiment to do. So Dr Rahman and his colleagues did it. Their subjects, men and women, gay and straight, were sat down one by one in a dimly lit room. The muscles that cause blinking were wired up with recording electrodes, and the subjects were fitted with headphones through which the sounds (sometimes a single startling noise, and sometimes a combination of soft and loud) were fed.

In the latter case, as compared with the former, straight men had blinks that were 40% less vigorous. In the case of straight women the drop was 13%. Lesbians dropped 33% which, statistically, made them more similar to straight men than straight women. Gay men were also intermediate, although in their case the difference was not statistically significant. Even in this apparently trivial matter, it seems, lesbians have male-like brains. So what is going on?

By default, people are female. Without the influence of testosterone in the womb, a fetus will develop into a girl. The way testosterone acts to turn a fetus male is still poorly understood. It seems likely, though, that different organs respond independently to the hormone, and may do so at different times. Hormonal surges at critical moments could thus cause particular organs in an otherwise female body to become “male”. (A lull in hormone production might have the opposite effect.) If the organ concerned is the brain, the result is more male-like behaviour including, possibly, male-type sexual preferences.

Previous research has provided some evidence for this idea. Lesbians, for instance, are more accurate throwers of objects such as darts than straight women. In this they resemble straight men in a way that has nothing to do with sexual preference. And tissues other than the brain's may be affected, too. On average, lesbians have ring fingers that are longer than their index fingers, a feature that is typical of men but not of heterosexual women. In that context, a difference in the blink of an eye is no surprise at all.

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