Evidence has shown that low birth weight is not a good thing for newborns. It’s associated with higher infant mortality and negative impacts on long-term outcomes, like lifelong income and cognitive ability.

But with improved medical practices, there’s a chance that this could be changing. A group of researchers in England, Germany, and Finland took a look at some demographic data and found that the link between birth weight and cognitive ability may have drastically declined over time—although it hasn't disappeared entirely.

The difficulty with research into questions like these is that it often takes crazy amounts of time. If you want to find out how birth weight affects people in adulthood, you need to track them from the moment they’re born and for decades afterwards. A lot of our evidence about the effects of birth weight had come from studying groups of people who were born in the 1950s.

Since then, there have been important medical developments, like the introduction of neonatal intensive care, as well as the use of assisted ventilation and drugs to treat babies who were born prematurely. “These advancements in medical care helped to prevent brain damage and other negative consequences often associated with low birth weight,” the researchers write. If the long-lasting traits that come with a low birth weight are partly due to medical problems, and we’re better at dealing with those medical problems, the outcomes should be changing, too.

In light of the medical improvements we've made, it seems to be high time for the evidence to get an update.

The researchers compared data from three different cohort studies, which use regular surveys and tests to follow people’s lives from birth onward. The first cohort followed 17,500 people who were born in 1958, the second followed 17,000 people born in 1970, and the Millennium Cohort Study (MCS) followed 19,000 people born between 2000 and 2002. The data included information on birth weight, details on the families, and tests of verbal cognitive ability (which correlates well with general cognitive ability) from around the age of 10 or 11.

In the 1958 and 1970 groups, people with low birth weight (below 2.5 kg) had substantially lower average scores for cognitive ability than groups of people with normal birth weight. In the MCS, the difference was still there, and it was still highly statistically significant—but it had dropped by more than half. The results are all the more striking because children who survived in the 2000s would probably have died in the 1950s and 1970s, meaning that the MCS group had an even higher proportion of babies with an extremely low birth weight.

Of course, it’s not just medical care that has changed over the past five decades. Other social changes, like education, reduction in poverty, and recommended behaviors during pregnancy (like not smoking) have all probably played a role.

The researchers built a wide range of social factors into their statistical tests and found that the factors did have some effect on the differences between the low-weight and normal-weight groups. After those effects had been accounted for, the gap between the normal-weight groups and low-weight groups narrowed a little. Other than that, the results looked similar: cognitive ability scores were still much lower for low-weight groups compared to normal-weight groups, and the difference was more than halved in the MCS cohort compared to the older groups.

It’s important to note that none of this research suggests that people with a low birth weight are "stupid." As with any other characteristic that you measure across a group of people, most people will fall somewhere around the middle, and there’ll be a few people at each extreme. In other words, you’ll get some low birth weight people with really high scores and some normal birth weight people with really low scores.

What it does mean is that the average score for the two groups is different, which suggests that something about low birth weight seems to be holding people back. Figuring out what it is could help to close the gap even further.

This finding leaves a number of questions to be answered. To begin with, it would be good to get a second recent cohort to ensure that the trend itself is real. The analysis looks only at one small segment of the global population, so research from other cohorts in other countries would contribute to a growing picture. They also only looked at cognitive ability tests given to children, so the results don’t tell us anything about what differences might persist into adulthood. And perhaps most importantly of all, there’s the question of what causes the remaining gap in cognitive ability and whether anything can be done about it.

PNAS, 2016. DOI: 10.1073/pnas.1605544114 (About DOIs).