Birth weight has significant and lasting effects, a new study finds. Weighing less than 5.5 pounds at birth increases the probability of dropping out of high school by one-third, reduces yearly earnings by about 15 percent and burdens people in their 30s and 40s with the health of someone who is 12 years older.

The study, presented May 22 in Washington, D.C. at the National Summit on America's Children, is the first to link birth weight with adult health and socioeconomic success using a full, nationally representative sample of the U.S. population. It is based on an analysis of more than 35 years of data on more than 12,000 individuals from the Panel Study of Income Dynamics, conducted since 1968 by the University of Michigan Institute for Social Research (ISR).

Funded by the National Institute on Aging, the analysis includes data from the original study families, plus their descendants who have gone on to form families of their own. Because of the study's unique genealogical design, the researchers were able to compare outcomes for siblings to isolate the impact of low birth weight apart from other common family conditions siblings share.

According to the authors, economists Rucker Johnson at the University of California, Berkeley, and Robert Schoeni at U-M, the study provides the most detailed look to date at how well-being and disadvantage are transmitted across generations within families.

"The poor economic status of parents at the time of pregnancy leads to worse birth outcomes for their children," Johnson and Schoeni write in a working paper from the U-M National Poverty Center. "In turn, these negative birth outcomes have harmful effects on the children's cognitive development, health, and human capital accumulation, and also health and economic status in adulthood. These effects then get passed on to the subsequent generation when the children, who are now adults, have their own children."

Among the key findings:

  • Compared to their normal birth weight siblings, low birth-weight children are 30 percent less likely to be in excellent or very good health in childhood. They also score significantly lower on reading, passage comprehension, and math achievement tests. Low birth-weight children are roughly one-third more likely to drop out of high school relative other children.
  • Low birth weight has significant negative effects on adult health, equivalent to being 12 years older in one's 30s and 40s. Weighing less than 5.5 pounds at birth increases the probability of being in fair or poor health as an adult by over 70 percent. Not only does birth weight have large and lasting effects across the life course, the researchers note, but its effects become larger later in life. For example, low birth weight children are nearly twice as likely as their normal birth-weight siblings to be in problematic health by ages 37-52 (23 percent versus 12 percent).
  • The earnings penalty for being born low weight also increases with age, from 10.2 percent at age 25 to 15.6 percent at age 35. Low birth weight is linked to a 10 percent reduction in hourly wages from ages 18-26, compared to the wages of normal birth-weight siblings, but a 22 percent reduction in wages from ages 37-52. Low birth-weight children, relative to their normal birth-weight siblings, work 7.4 percent fewer hours in adulthood. These effects of poor infant health persist, in sibling comparisons, after accounting for the independent effects of birth order, mother's age at birth, birth year cohort, race/ethnicity, family structure, parental income, and parental fertility timing.
  • Not only does low income and lack of health insurance during pregnancy increase the likelihood of poor birth outcomes, but limited parental resources also influence the lasting impacts of poor infant health. The absence of health insurance during childhood intensifies the negative impact of low birth weight. For example, the harmful effects of low birth-weight on adult labor force participation is over twice as large if the adult did not have health insurance in childhood. Additionally, the harmful effects of low birth weight on adult health are 2.7 times larger for those who were uninsured in childhood.
  • Among the poorest families (those with incomes of less than $15,000 in 1997 dollars), increasing income by $10,000 lowers the probability of low-weight birth by 2.18 percent. Increasing income by the same amount among lower middle income and high-income families does not significantly influence birth weight. Moreover, while parental income during pregnancy is beneficial at the low end of the income distribution for all newborns, the income effect is much larger for infants who were predisposed to be born low weight because their mothers were themselves born low weight.
  • The large racial differences in adult health status through mid-life in the U.S. can be fully explained by a few early life factors---birth weight, parental family income and health insurance coverage.

The ISR Panel Study on Income Dynamics is funded primarily by the National Science Foundation, the National Institute on Aging, and the National Institute on Child and Human Development. The study is co-directed by Schoeni and U-M economist Frank Stafford.