> what does "50% heritable" even mean? It means that if you plotted a graph of orphaned children IQs on the X axis and their biological parents IQs on the Y ax

> what does "50% heritable" even mean? It means that if you plotted a graph of orphaned children IQs on the X axis and their biological parents IQs on the Y axis you'll get a messy and very bulgy line going up and to the right. And this kind of thing is discovered by noticing that the IQs of identical twins are much more similar than those of non-identical twins. Wikipedia has a fairly good overview: https://en.m.wikipedia.org/wiki/Heritability_of_IQ https://en.m.wikipedia.org/wiki/Heritability_of_IQ