Breastfed babies have higher IQ and income in adulthood, study finds

The benefits of breastfeeding appear to last long after a baby grows up -- and go far beyond better health.

A new study finds children who were breastfed for at least 12 months went on to have higher IQ scores as adults, completed more schooling and earned higher salaries.

Doctors have known for years that infants who are breastfed perform better on intelligence tests in childhood, but the new study, published in The Lancet Global Health, makes the strongest link yet to IQ in adulthood.

For the study, researchers followed more than 3,400 children in Brazil for a period of 30 years.

CBS News medical contributor Dr. Holly Phillips says the results were striking. "It turns out, compared to kids who were breastfed for less than a month, kids who are breastfed for a year or longer had almost 4 points higher in IQ" in adulthood, she told "CBS This Morning."

The children who were breastfed also stayed in school about a year longer and earned more money when they grew up -- an amount equal to about $104 extra each month.

The researchers corrected for other factors that could influence IQ, such as the child's birth weight, mother's education level and family income, and concluded breastfeeding makes a significant difference.

Previous studies that suggested breastfeeding may boost adult IQ focused on higher-income families and researchers weren't sure the results would hold up in a more diverse population, but the Brazil study found the connection is real. What's less clear is the reason why.

"The question is always, is it the breast milk itself or is it the breastfeeding that conveys the benefit?" Phillips said. "I think it's both. We do know breast milk has a number of nutrients that support the developing brain and the nervous system. On the other hand, the communication and bonding that happens during the breastfeeding process changes the brain physically, it makes neurons connect with one another. That increases IQ and it also boosts confidence, which translates into achievement later in life. So I think are a number of factors that are contributing here."

Children who are breastfed tend to enjoy a number of other health benefits. They are less likely to suffer from a variety of gastrointestinal and respiratory infections, asthma, diabetes and other conditions. Studies have also shown they have better thinking and memory skills and score higher on cognitive, language and motor development tests as toddlers.

But Phillips urges new moms and dads not to stress out if breastfeeding isn't working for them. Nurturing parents can still raise happy, healthy children without it.

"I see a lot of women in my practice who feel a great sense of shame or guilt if they can't breastfeed for a year," Phillips said. "They should know they're not alone. Only 27 percent of women in America breastfeed for a year. You know the number one reason why, the workforce. Not very many jobs are conducive to doing that for a year. So don't beat yourself up. Do what you can. Know the benefits, and that's it."