You sure didn't read thoroughly; the next sentence after that one explains it. They were comparing monozygotic and dizygotic twins, which allowed them to measu

You sure didn't read thoroughly; the next sentence after that one explains it. They were comparing monozygotic and dizygotic twins, which allowed them to measure to the extent that they were similar because of being raised in the same family at the same time. Comparing monozygotic twins raised in different families is another way to measure hertibility. Both have their issues, of course. But measures of the heritbility of personality generally do hover around .5 _+/- .1 in other studies.