Thread by @WiringTheBrain

A little thread on the inevitability of heritability of behavioral or psychological traits 🧵⬇️

Different species of animals - including humans - vary in their innate "natures" (their behavioral and cognitive tendencies and capacities)

These tendencies and capacities are somehow embodied in the structures of their brains and bodies, based on a program encoded in their DNA

Genetic variation is a fact of life. Errors arise every time eggs and sperm are generated, introducing new genetic variants. Ones that are tolerated survive and accumulate.

The developmental programs in animal genomes have evolved to be robust to such variation, partly by buffering its effects but also by tolerating its effects on resultant phenotypes

Most phenotypic outcomes can vary over a range - bit shorter? bit taller? that's okay, you can still survive. Which is just as well cuz genetic variation can't be avoided and its effects can only be buffered up to a point

So all species accumulate genetic variation that leads to heritable differences between individuals in all kinds of traits (that have a genetically specified species 'norm')

Human psychology is no exception. It is the product of an incredibly complex program in the canonical human genome that specifies a canonical human brain

Except no individual has the 'canonical' human genome or brain - we all have inevitable variations in our genomes and *consequent variations* in our brains and our psychological traits

If human nature is encoded in our DNA generally, there is no way such individual variation could not also occur. It is the flip side of the same coin.

And if genetic differences make people psychologically different from each other, then related people (like twins or family members) will be more similar to each other

Which is exactly what decades of twin and family studies find and (controlling for shared environments) why they can be used to estimate the "heritability" of such traits

So, even with all the possible methodological quibbles with particular study designs, the fundamental observation - that genetic differences affect our individual natures - is completely expected. It couldn't be otherwise.