White supremacists like to claim that twin studies prove that black people are less intelligent due to genetics. Apparently Sam seems to think that Democrats refusing to accept the shoddy science behind twin studies (the conclusions of which are often used in service of racist meta-studies) somehow implies that they are also "anti-science". Probably best to just ignore the comment.
Twin studies are mostly used to show that traits are heritable at an individual level.
Edit: It strikes me that the strawman argument you've constructed is a perfect example of left-wing science denial. The endemic use of the technique in fields like psychology, criminology, biology, is something you prefer to ignore.
I wouldn't agree with that either, but most people would, so might as well stick it in there.
Let's say it's part of the causal chain, but also your cells don't even have the same genome, it mutates over time, women integrate cells from their children, so it gets messy.
Yup. "Scientific" racism has a long history. Some of the various Confederate declarations of secession claim natural inferiority as a cause for slaveholding.
Edit: It strikes me that the strawman argument you've constructed is a perfect example of left-wing science denial. The endemic use of the technique in fields like psychology, criminology, biology, is something you prefer to ignore.