Breastfed babies may grow up to have higher intelligence and make more money than their formula-fed peers. That was the conclusion of a new study out of Brazil, and published in The Lancet this week, which found positive associations between breastfeeding, higher IQ, longer schooling, and higher adult income among a group of babies tracked for 30 years.

There's a lot to unpack in a study this ambitious. And before we get into the details, it's worth noting that we already know that breastfeeding is good for babies—breast milk is full of protective nutrients for infants at a time when they're especially vulnerable. The American Academy of Pediatrics recommends mothers breastfeed their babies for at least the first six months of life. (There's also enormous pressure on moms to breastfeed, so much that the choice has been politicized far beyond what may be best for individual babies and their moms, but that's another story.)

What may be most telling about the findings in the recent Lancet study is the contingency of babies who dropped out of the research group over time. The rate of what clinical researchers refer to as "loss of follow-up" is relatively high, which raises a host of questions about the validity of the study's conclusions.

Researchers began with a group of 5,914 newborn babies in 1982, and were able to follow-up with 3,493 of them three decades later. That's a follow-up rate of 59 percent. (Researchers consider their follow-up rate to have been 68 percent because they count 325 subjects who are known to have died and several for whom they didn't have complete IQ or breastfeeding information.) Though one of the study's co-authors, Bernardo Lessa Hort, characterized the rate as "quite good," many scientists would say it's suboptimal.

In general, a follow-up rate has to exceed 60 percent to be considered acceptable, and reach 80 or 90 percent for it to be considered good. "A good rule of thumb," wrote the author of a 2011 study about loss to follow-up in clinical trials, "is that less than 5 percent loss leads to little bias, while more than 20 percent poses serious threats to validity."

Even with small rates of loss, significant biases are possible. And the number of people who drop out of a study is actually less important than how those departures are related to the findings. The contributing factors that make someone lost to follow-up are often the same factors that would affect the outcome of the study.

In the context of a study about breastfed babies, the obvious example is socioeconomic status. That's because researchers tracked key socioeconomic markers—income levels, duration of schooling—that could also influence whether a person is findable for a follow-up. So, in theory, the breastfed babies who wouldn't have ended up earning as much or performing as well on an IQ test could have been the ones lost to the study. Which is why these sorts of losses to follow-up are considered one of the main threats to validity in clinical trials, and they rarely occur randomly. "The only way to ensure that losses to follow-up have not biased study results," wrote the authors of a 1998 study about tracking research cohort, "is to keep all losses to an absolute minimum."

Even among the study subjects whom researchers were able to follow for the full 30 years, there are questions. Like the fact that mothers participating in the study were required to recall their breastfeeding habits years after their babies were born. Researchers first followed up with them in 1984, two years after enrollment in the study.

Relying on a research subject's memory can be a major threat to validity, and breastfeeding mothers have provided inaccurate estimates to scientists before. "When mothers are asked retrospectively in breastfeeding studies about breastfeeding duration, the reports become increasingly inaccurate with increasing time since cessation," according to a 2008 paper about research biases by Lars Fadnes. "Mothers who had breastfed their infants for longer systematically underreported their breastfeeding duration; while in contrast, those who had breastfed for a shorter time over-reported the duration."

And even if breastfed babies really do grow up to have higher IQs and make more money, how do we know it's because they were breastfed? "Breastfeeding is positively associated with socioeconomic standards," acknowledged Horta, one of the authors of the Brazil study, in a podcast conversation with The Lancet about his work. "So there is always a question of whether [an outcome like higher IQ] is a consequence of breastfeeding by itself... The kids who are breastfed are wealthier."

In other words, because mothers who breastfeed are also more likely to enjoy a higher socioeconomic status, maybe the breastfed babies have higher IQs and better income in life because of advantages—other than breast milk—associated with that higher socioeconomic status. Does the extra $100 a month that breastfed babies apparently end up making, according to this study's findings, really come form the fact that they were breastfed—or from other advantages they had over time? Correlation, as they say, is not causation. (Researchers said they took into account household assets and several other confounding factors that might bias the results of the study, but their methodology is not clear from what's published.)

There's also the larger question of what an association between breastfeeding and higher IQ points—if there truly is one—might practically mean. Researchers found that babies who were breastfed longer grew up to perform about four points better on IQ tests. Which raises a question of what four IQ points—in either direction—can really do for a person. Performing well on such an exam may be a better indicator of test-taking skill rather than a factor that contributes to larger success. Measuring either—intelligence or success—is tricky because, like happiness, they're difficult to quantify in meaningful terms. And though breastfeeding may play a role in how a kid turns out, 30 years of parenting surely has more to do with what kind of adult a baby becomes.