The differences the researchers noticed were in relation to children born in better socioeconomic conditions. So it is a relative difference. That poverty and related aspects such as malnutrition, potentially common in medieval times will affect the brain is clear. But as conditions improve, as they have over the centuries, so will the average brain — this may also be a major contributing factor to the Flynn Effect whereby average IQ has been increasing since records began.

The point I’m making though is that some babies are born with a relative disadvantage and this will impact their future development, ability, and opportunity. And this is because of the conditions of their pregnancy — they have no choice or way to influence this.

That is not good for the individual but also not good for society.