Heritability is not a fundamental quantity, and it often does not correspond to what is expected.
I found that a useful example for understanding this is consideration of the number of arms that a person has. This seems like it would be 100% heritable. You have two arms because your parents have two arms. This would be true if the group being analysed includes not only people but also other animals such as worms, which have zero arms because their parents have zero arms.
However, normally in these discussions only humans are considered and thus genetic differences play no part in how many arms you have. All of the people that don’t have two arms are in that situation because of environmental effects. These effects include amputations and birth defects, such as those caused by thalidomide. Thus the heritability of the number of arms that humans have is actually zero.
A counter intuitive result of this is that if an ideal world means that everyone is exposed to a uniformly high quality environment, but there is a lot of genetic variation because we don’t know how to genetically engineer humans yet, then heritability of beneficial traits such as IQ would be quite high, because the environmental variation would be suppressed.
Heritability is not a fundamental quantity, and it often does not correspond to what is expected.
I found that a useful example for understanding this is consideration of the number of arms that a person has. This seems like it would be 100% heritable. You have two arms because your parents have two arms. This would be true if the group being analysed includes not only people but also other animals such as worms, which have zero arms because their parents have zero arms.
However, normally in these discussions only humans are considered and thus genetic differences play no part in how many arms you have. All of the people that don’t have two arms are in that situation because of environmental effects. These effects include amputations and birth defects, such as those caused by thalidomide. Thus the heritability of the number of arms that humans have is actually zero.
A counter intuitive result of this is that if an ideal world means that everyone is exposed to a uniformly high quality environment, but there is a lot of genetic variation because we don’t know how to genetically engineer humans yet, then heritability of beneficial traits such as IQ would be quite high, because the environmental variation would be suppressed.