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 reduced 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.