The reason we’re like our parents is that we share half of their DNA. Estimates vary but it’s generally agreed that the portion IQ variance which is accounted for by our genes rises from about 50% in childhood to around 80% in adulthood. And the proportion that is due to our shared environment (parenting) starts at about 30% and washes out almost entirely by adulthood. At the same time, other environmental effects (usually referred to as ‘non-shared environment’) become increasingly important, with education and peer groups taking over the role of parents.
If two adopted children are brought up in the same home they will be no more similar in personality than two adopted children raised in completely different homes. Similarly, a pair of identical twins reared in the same home are not much more alike than twins reared in separate homes. As everyone knows, identical twin share 100% of their genetic material,* but despite this there are often observable differences in their behaviour and personalities. What accounts for these differences is referred to as ‘the environment’. Behaviour geneticists have identified two broad categories of environmental effects: shared and non-shared. Shared environmental effects are those that siblings have in common and, by definition, these cannot account for the differences between identical twins because if they are raised in the same family they share them. Therefore, all differences must be non-shared in origin. Essentially these are environmental factors that are unique to each twin. Possibly they have different friends, maybe they’re in different classes at school and so have different teachers and so on.
This is not to say that parenting has no impact – clearly it has a huge impact on the environment a child experiences – but this doesn’t account for the differences between people brought up in that environment. This runs counter to every parent’s lived experience. We see our children ape our mannerisms and take on our beliefs and values – how could this not be the result of parenting? As with accent, so with religious belief: we only retain the beliefs of our parents to the degree that they are not in conflict with those of our peers.
Since 1979, the Minnesota Twin Family Study has been tracking down and reuniting pairs of separated twins from all around the world and collecting data on their personalities and IQs. Other studies have focused on comparing the IQs of adopted children to their adoptive parents. Taken together, these studies present an intriguing picture. The IQ score of the same person taking a test on different days would produce a correlation of about 0.87. The scores of identical twins raised together produce a correlation of 0.86. This suggests that the IQ of identical twins brought up in the same family is pretty much indistinguishable. So far, so predict- able. But the scores of identical twins raised apart produce a correlation of 0.76. When compared to fraternal twins raised together (0.55) and biological siblings (0.47), this is impressive. The scores of parents and children living together is 0.4, whereas parent and children living apart is 0.31. Not much different. The real shock is that there is no more correlation between adoptive siblings than there is between any two unrelated people living apart.
While shared environmental effects are often strong during childhood, they wear off by adulthood. This makes some sense. After all, children have little control over their environments when they’re at home. But as soon as they start school, alien mannerisms and foreign values begin to enter a child’s repertoire. It’s only by the time we reach adulthood that we are free to behave exactly as we please (except perhaps when we return to visit our parents).
This paper by Thomas J. Bouchard, Jr. summarises much of the research into the role of genes on human behaviour and finds that, “Shared environmental influences are often, but not always, of less importance than genetic factors, and often decrease to near zero after adolescence.”
As you can see in the above table, shared environment effects are very low or non-existent for most traits in adults. Aha! – you might be thinking – it looks like parenting can affect ‘religiousness’ to a moderate degree, and look, various other traits show some small effects. Clearly the claim that parenting has no effect is obvious bunkum.
Well, that could be the case, but these small estimates are often equally attributed to measurement errors and unaddressed confounding factors. It’s also true that if children experience extreme neglect or abuse (espe- cially before the age of 5), parenting can cause lasting damage and often results in permanent cognitive impairment. While parents should not expect their values or beliefs to be adopted wholesale by their children, we naturally have a duty of care. Making sure they eat their vegetables, brush their teeth and go to school may not result in habits that persist into adulthood, but they make for a healthier, happier childhood. The effects of parenting on IQ seem particularly conclusive. Shared environ- mental effects decrease steadily from 0.54 at age 5 to 0.26 at age 10, before disappearing entirely. The good news is, Larkin was wrong!
In her book The Nurture Assumption, Judith Rich Harris presents three key propositions:
- Parents have little or no power to shape their children’s personalities
- Children are socialised, and their personalities shaped, by the experiences they have outside the home, in the environment they share with peers.
- The tendency for an individual to behave in a somewhat similar fashion in different social contexts is due, in most cases, to a genetic predisposition to behave in that manner.
The evidence she amasses in support of these elegantly constructed propositions is truly impressive and, for those who remain unconvinced by my poor arguments, you really should give this eminently readable book a go. For those who unwilling to undertake such a chore, just read Appendix 2 in the link above (p 357 – 369). If you’re short on time, it might even be worth just reading Chapter 2, The Nature (and Nurture of the Evidence.
So, what is the point of parenting? Well, as the proud father of two wonderful daughters if you really need to ask, then I worry for you. Surely there doesn’t need to be a point for us to do the best for our children? The point, if there is one, is that being nice to your kids improves the quality of their childhoods. And, if you need further incentive, the one enduring effect of parenting is the quality of the relationship you will have with your adult children – the more effort you make to be kind, fair and supportive, the more likely your children are to like you and the less likely they are to dump you in a retirement home.
Harris ends her book with these words:
The bond between parent and child lasts a lifetime. We kiss our parents goodbye not once but many times; we do not lose track of them. Each visit home gives us opportunities to take out family memories and look at them again. Meanwhile, our childhood friends have scattered to the winds and we’ve forgotten what happened on the playground.
When you think about childhood you think about your parents. Blame it on the relationship department of your mind, which has usurped more than its rightful share of your thoughts and memories.
As for what’s wrong with you: don’t blame it on your parents.
I understand that all this is hard to accept. I understand that sharing this sort of research directly contradicts people’s cherished beliefs about the way the world works and I get that the power of cognitive dissonance makes some people want to lash out, nit pick and hurl insults. Fine. But please try to bear in mind that I’m not trying to upset you or devalue your experience. If you want to get angry, read Harris’s book rather than attacking this blog. If you’re unconvinced, so be it. This tweet sums up the problem beautifully:
As a parent, I don’t really want to believe this. As a son, it’s quite easy to accept.
— Nick Wells (@NSMWells) August 11, 2017
Those looking for more might enjoy this post does a great job of bringing together huge swathes of research to address the claims of Tiger Mom, Amy Chua.
* Except for a few minute mutations.