You are what your mother fed you, especially if you are a young man. A new study that looked at the height, weight, muscle mass, strength, testosterone levels and even sexual history of 770 Filipino men tracked from birth reveals that the degree of a baby boy's growth in the first six months of life predicts the extent of his masculine characteristics.

This period is crucial because it is "when testosterone is at roughly adult levels," explains biological anthropologist Christopher Kuzawa of Northwestern University, who led the study published online September 14 in Proceedings of the National Academy of Sciences. "Men who as babies gained weight rapidly during the period of this testosterone surge matured earlier, were taller, had more muscle, were stronger and had higher testosterone levels. Because they matured earlier, not surprisingly, they started having sex for the first time at a younger age and had more sex partners."

That's pretty much the recipe for success from an evolutionary standpoint, and it also provides evidence as to how nurture might shape an underlying nature. After all, this testosterone surge in early life contributes to the male characteristics of many mammals. "The things that define males are flexible characteristics in response to nutrition," Kuzawa argues. "Your fate is not hardwired."

Kuzawa and his colleagues conducted follow-up interviews with 770 Filipino men whose mothers had enrolled in 1983 in the Cebu Longitudinal Health and Nutrition Survey, a cross-section of the population of the Philippine's second-largest city. The researchers looked at growth data from the boys' first two years of life as well as information collected when they were eight, 11 and 14. Finally, the researchers interviewed the men in their early 20s.

Those boys who had grown the fastest from birth to six months, who generally were also the boys who were breast-fed and grew up in wealthier homes, became the men exhibiting the most masculine characteristics. A similar analysis of 690 Filipino women revealed no such difference (Kuzawa is now conducting a follow-up study to see if there are any long-term effects of early nutrition on the birth weight of these women's offspring.)

That discrepancy could be because the testosterone surge literally shapes the bodies of men, including organs and bones. The "environment" the baby boys encounter—most important, the nutritional environment—then governs the size of the surge. "The slow growers are undernourished and nutrition-stressed," Kuzawa notes.

Of course, the researchers could simply be demonstrating the effects of nutrition on male development—or even the effects of breast-feeding. "There may be factors in the mother's breast milk, but we don't have the ability to actually look at that," Kuzawa notes. But he adds that the effect in humans matches the same biological pattern found in other animals, such as rats. "If nutrition is better, they can afford more of those traits, and under more limiting nutrition circumstances it might benefit that individual not to commit to that kind of costly body."

But the effects of socioeconomic class and culture are hard to rule out as well. The richest boys also grew fastest throughout the entire first two years. "We are seeing overwhelmingly an effect of poverty as an influence on outcomes," Kuzawa says, although cautioning on extrapolating from one specific set of circumstances in the Philippines to any other country. "We can't predict how these findings play out in a population," he says of the U.S., for example, "where the fastest growers are actually gaining too much weight."