Social scientists have long argued whether environment or heredity is more important in determining an individual's behavior, intelligence, and personality. Now, an economist from the University of California at Berkeley says he has found a strong environmental determinant to a person's adult health and success. Rucker Johnson says they're both tied to birth weight.

Johnson looked at 40 years of data from a long-term survey conducted in the United States. He based his analysis on an idea called the fetal origin hypothesis. "[It suggests] when nutritional intake of a fetus is limited, that the body's physiology and metabolism would be fundamentally altered in ways that would manifest much later in life, in the form of increased risk of hypertension, stroke, heart disease. And more generally, that health would deteriorate much more rapidly into old age."

Johnson compared poor children born weighing less than 2.5 kilos to their siblings who were normal birth weight. That way, each group of subjects had the same family background and experiences, and were easy to compare. "What we found was that not only were they having reduced levels of cognition in terms of math and reading and passive comprehension scores in early childhood, but we also found that they had worse health in childhood and they had lower levels of high school graduation."

He also found that the health effects of low birth weight extended into adulthood. "It's equivalent to being 12 years older in one's 30s and 40s," he explains. "That is, the low birth weight child is reaching a level of health deterioration rate 12 years prior to the normal birth weight sibling in one's 30s and 40s."

He found that children from families that were better off, who had access to health care and educational help, overcame the effects of low birth weight.

Johnson says this research points to the need to create policies that focus on improving the lives of poor youngsters. He says that may be the only way to overcome the combined effects of environment and heredity that he says play a role in the intergenerational continuation of poverty in some families.

Johnson presented his research at a national poverty conference held in Washington, D.C. last month and he says an article is under review at an economics journal  for publication later this year.