The empirical data show it remains very much true today in modernized societies. IF both your parents are Dems, you are 5 times more likely to become a Dem than a Republican as an adult, and the inverse if both parents are Republicans. Split party parents have kids that are more 50/50 but they tend to lean toward their mom's party. When kid's don't adopt their parent's party is usually because they merely go "independent" rather than to the opposing party. Note that this is true, even when the kid (wrongly) believes and claims that his parents had little to no influence on their own politics. Also, it isn't just nurture but genes and biology. This is growing evidence that biology plays are significant role is shaping ideology on all issues, including politics.
Thus, even if given up for adoption, a baby with conservative biological parents is more likely to grow up to be a conservative.
So what am I?
My parents were Republicans in the time before the neocons and the Tea party. Their politics didn't shift but the parties did--they ended up Democrats.