A new study suggests a strong but controversial link between breast-feeding, intelligence and wealth. Doctors not involved in the study, however, suggested other factors such as genetics and social class have stronger influences on these outcomes.

The study by a team of researchers from the Federal University of Pelotas in Brazil shows that breast-fed babies might be smarter, better educated and could become richer as adults. It was published March 17 in The Lancet Global Health.

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"Breast-feeding is associated with improved performance in intelligence tests at age 30 and also has an important effect on a societal level by increasing educational attainment and income in adulthood," said lead researcher Dr. Bernardo Horta.

"Taking into consideration these long-term benefits of breast-feeding as well as its short-term consequences, it is important to get more women to breast-feed their infants," he added.

Horta's team collected decades of data on nearly 3,500 infants born in 1982 in Pelotas, Brazil. The data included information on breast-feeding, education, income and standard IQ test results at age 30.

They found that breast-fed infants had higher IQs, more years of school and higher incomes as adults than those who weren't breast-fed.

They also discovered that the longer a baby was breast-fed, the greater these benefits.

An infant breast-fed for at least a year had an almost four point increase above the average IQ as an adult. Infants breast-fed for a year attained almost an extra year of schooling compared to participants breast-fed less than one month, said the study.

Intriguingly, researchers found income was about one-third higher than the median in participants that had been breast-fed for at least a year.

Horta believes breast-feeding's impact on intelligence might stem from the saturated fatty acids in breast milk necessary for brain development. He also suggested the amount of breast milk an infant consumes may play a role in increasing IQ.

Breast-feeding, however, wasn't more common among highly educated, high-income women but was evenly distributed by social class in this study, said Horta.

Researchers said they compensated for social and biological variables that might have contributed to increases in IQ. Those variables included family income, parental schooling, genes, a mother's smoking during pregnancy, the mother's age, birth weight and type of delivery.

Some doctors, however, issued warnings about these apparent benefits. Dr. David Mendez, a neonatologist at Miami Children's Hospital, said parents should not take home the message that "if you do not breast-feed, your child will not be a successful adult."

"Clearly, to tie a singular event such as breast-feeding to the future potential of an adult is problematic," Mendez said.

"This study really points out the benefits of a nurturing environment, the energy and effort that parents put into successfully breast-feeding their children, which is indicative of parents who are loving and caring. That environment will help a child to reach their full potential as an adult."

Dr. Erik Mortensen, a pediatrician not involved with the study, said it's impossible to prove breast-fed babies grow-up to be smarter, more successful adults.

He said the study only shows an association between breast-feeding and IQ, not proof of cause and effect. But the link is very strong, according to Dr. Mortensen, who works at the department of public health at the University of Copenhagen in Denmark.

It's important for women to know that breast-feeding does not determine a baby's destiny, he added.

"It may not be a causal effect, but in my judgment, the overall evidence from other studies and the Brazilian study suggests that the effects are causal," he said.

"Breast-feeding is only one of many factors which influence the development of intelligence. And it actually appears that factors such as parental intelligence, parental education and social class have a stronger influence on the development of offspring intelligence," he said.