Having a college educated mother increases an adoptee’s probability of graduating from college by 7 percentage points, but raises a biological child’s probability of graduating from college by 26 percentage points. In contrast, transmission of drinking and smoking behavior from parents to children is as strong for adoptees as for non-adoptees. For height, obesity, and income, transmission coefficients are significantly higher for non-adoptees than for adoptees. In this sample, sibling gender composition does not appear to affect adoptee outcomes nor does the mix of adoptee siblings versus biological siblings.
Factors like religious affiliation and linguistic accent are also very strongly cultural. Nobody is born Christian or Buddhist, or speaking Spanish or English. Genetics may influence you attitude towards religion, or your likelihood of having a learning disability related to reading, but it won't determine the content of your religious beliefs or the language that you will learn.
But, parental income at the time of adoption has virtually no effect on adopted child income. Parental education does have a modest effect on adopted child educational level (parents who went to graduate school have adopted kids with half a year more of education than those who merely graduated from high school), but the effect is really strong only for parents who don't have at least a high school diploma (those kids get dragged down). In contrast, parental income and education have a rather pronounced effect on the the incomes and educational levels of biological children suggesting that these outcomes are mediated by factors like hereditary IQ and personality effects.
This doesn't necessarily mean that your parents alone are your destiny. Quite extensive studies have shown that there is a fairly wide range of stable differences in all sorts of traits between even full blooded siblings on factors from IQ to personality and that those differences impact life outcomes. Almost no intelligence or personality or mental health traits are 100% hereditary or even much more than 85% hereditary, and most hover at the 50% level or less.
There is also a fair amount of evidence to show that really bad parenting and early childhood deprivations or physical traumas can screw anyone up no matter how much potential that person may have had at birth. Genuine poverty, malnutrition, lead poisoning, prolonged isolation, and abuse are all bad for kids no matter how promising they are (although there are even genes that govern how well kids can cope with some of these adversities, a trait sometimes described as resilience).
But, given the intense amount of effort the parents put into deciding precisely how to raise our children, parenting choices make less of a difference than one would intuitively expect. Princes raised by peasants really do generally grow up to be princes, royal bastard are going to tend to outperform their ordinary siblings, and kids raised in circumstances far more favorable than those they were born in because they are adopted generally underachieve relative to the biological children of their adoptive parents and have less in common with their siblings than you would expect from a shared upbringing. It may have a ring to the monarchist propaganda that Disney likes to produce, but there is truth to it. The myth of Achilles, who was a prince raised by farmer parents as their own who rises to prominence, is closer to real life than the Prince and the Pauper.
Indeed, there are at least one or two studies out there that show that decent parenting from just about anyone is fine. A good step-parent or adoptive parent or extended family member in lieu of a parent can raise you well enough to reach essentially the same potential you could have if raised by a biological parent - or better if a biological parent's exceptionally bad parenting was factor that could have dragged you down.
It is hardly surprising that assortive mating should strength social class divides. If people who have what society rewards have kids with other people who have the same traits, and people who lack that have kids with other people who lack that, we expect that there kids are more likely to reproduce their status than in a society where couples form at random or are deliberately unequal in traits that society rewards, which will tend to produce more variability between siblings and more averaging out.
More paradoxically, each generation that society is meritocratic should strength class divideds in future generations. Some people from modest beginnings will always be more talented than their parents, and some people with talented parents will always have less of what made their talents successful. Regression towards the mean is to be expected, and the less talent matters the less talent will be associated with social class and the more vulnerable the social class system will be to being upset by talented people born in a lower socio-economic class usurping less talented people born to privilege.
By all accounts, American marriages are more assortive than they used to be, and we have had roughly two full successive generations of a relatively meritocratic society. The longer this continues, the more stark social class divisions can be expected to become, with really no obvious limits. A few centuries of carefully preserved meritocracy and assortive mating should paradoxically greatly reduce social class mobility and make the gradations within social classes more fine, to the extent that social class divisions are a product of individual personal traits rather than the nature of the slots the arise from our current technology and economic system. A hunter-gather society, for example, can support only so much social class stratification.
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