From Scientific American:
We all know the stereotypes: an unusually light, delicate, effeminate air in a little boy's step, an interest in dolls, makeup, princesses and dresses, and a strong distaste for rough play with other boys. In little girls, there is the outwardly boyish stance, perhaps a penchant for tools, a square-jawed readiness for physical tussles with boys, and an aversion to all the perfumed, delicate trappings of femininity.
These behavioral patterns are feared, loathed and often spoken of directly as harbingers of adult homosexuality. It is only relatively recently, however, that developmental scientists have conducted controlled studies to identify the earliest and most reliable signs of adult homosexuality. In looking carefully at the childhoods of gay adults, researchers are finding an intriguing set of behavioral indicators that homosexuals seem to have in common. Curiously enough, the age-old homophobic fears of many parents reflect some genuine predictive currency.