Fertility Treatments Increase Incidence Of Twins At A Cost; Babies Weigh Less At Birth And Earn Less As Adults

On average, twins weigh 28 ounces less at birth than children born singly, and, according to Penn researchers, this translates into a 12 percent reduction in lifetime earnings for twins compared to singletons.

Penn economics professors Jere Behrman and Mark Rosenzweig are the first researchers to document the causal link between birth weights and lifetime earnings. Their report, "The Returns to Increasing Body Weight," concludes that there are "real payoffs" to interventions that help pregnant women increase their babies' weight at birth.

In 1994 and 1995, the researchers collected data on female twins from the Minnesota Twins Registry, the largest birth-certificate-based twins registry in the U.S. It contains birth records of all twins born in Minnesota between 1936 and 1955. Their empirical research focused on 404 pairs of genetically identical female twins, studying their birth weight, schooling attainment, adult height and adult earnings.

Behrman and Rosenzweig found that twins have considerably lower average birth weight (5 pounds, 10 ounces) compared to the general population (7 pounds, 6 ounces). Also, almost half of twins are low birth weight by the standard definition of low birth weight (below 5 pounds and 8 ounces.)

According to Behrman, when potential parents undergo fertility treatments that could result in multiple births, they undertake a double risk. "Not only do they take the chance of having more children than they bargained for but of having children with less satisfactory education and economic prospects than would result from a singleton birth," Behrman said.

Increasing birth weight increases adult schooling attainment, adult height and adult earnings. Augmenting a child's birth weight by a pound increases the level of education the child achieves later in life by about a third of a year.

A one-pound increase in birth weight brought about by increased womb nutrients would increase adult height by more than half an inch. Augmenting a child's birth weight by a pound increases adult earnings by more than 7 percent.

While there are previous documented associations between birth weights and child development, those associations intermingle the effects of birth weight, genetics and family background. Children with poorer family background, for example, tend to have lower birth weights.

This study for the first time isolates the impact of birth weight while controlling for genetics and family background. It does so by considering how differences in birth weights for identical twins – who have the same genetics and same family background – relate to differences in their schooling and in their adult weight, height and earnings.