Big brothers. Study after study -- including one of 87,000 British men published last year -- has found that gay men have more older brothers than straight men do. Only big brothers count. Lesbians don't show such patterns.
The numbers: Each older brother will increase a man's chances of being gay by 33%, says Ray Blanchard of the University of Toronto, an expert on the "big-brother effect." That's not as dramatic as it might sound. A man's chance of being gay is pretty low to begin with -- perhaps as low as 2% (lowered from 10% by researchers in the early 1990s). So having one older brother ups the chance to only about 2.6%.
Hands. More lefties -- or at least more somewhat-ambidextrous folks -- crop up in the gay population than among straight people, several studies have shown. An analysis of more than 23,000 men and women from North America and Europe in 2000 found that being non-right-handed seems to increase a man's chances of being gay by about 34%, and a woman's by about 90%.
Fingers. Studies of sexual orientation have shown that lesbian women are more likely to have longer ring fingers, suggesting exposure to higher levels of foetal testosterone. Professor John Manning said research he had conducted suggested that gay men were more likely to display feminised finger ratios, suggesting less testosterone exposure in the womb.
Hair. A 2004 study of nearly 500 men -- 272 on Delaware's Rehoboth Beach, popular with gay men, 200 on a beach without that reputation -- found that hair on the heads of men on the gay beach was 3.5 times more likely to grow in a counterclockwise direction. (Scalp hair typically resembles a clockwise-rotating typhoon.)
Penis size. If exposure to testosterone in the womb influences sexual orientation, scientists reckon that straight and gay people would differ in body parts strongly affected by testosterone, such as the penis.
The numbers: Anthony Bogaert of Brock University in Ontario and his colleagues re-analyzed data on 5,000 gay and straight men from sexologist Alfred Kinsey's famous files, collected from the 1930s to the 1960s. The results, published in 1999, showed that gay men had longer, thicker penises than did straight men: on average, about 6.5 inches long and 4.95 inches around when erect, versus 6.1 inches long and 4.8 inches around for straight men.
Too bad we can't check the last part.