Shared environment not a factor

A 2000 paper summarized the evidence:

When genetic similarity is controlled, siblings often appear no more alike than individuals selected at random from the population. ...

Why Are Children in the Same Family So Different?

In what may have been the most influential article ever written in the field of developmental behavior genetics, Plomin and Daniels (1987) reviewed evidence that a substantial portion of the variability in behavioral outcomes could not be explained by the additive effects of genotype or the environmental influences of families. They suggested that this residual term, which they called the nonshared environment, had been neglected by environmentally oriented researchers who assumed that the most important mechanisms of environmental action involved familial variables, like socioeconomic status and parenting styles, that are shared by siblings raised in the same home and serve to make siblings more similar to each other. Indeed, Plomin and Daniels argued, once genetic relatedness has been taken into account, siblings seem to be hardly more similar than children chosen at random from the population.

An important indicator of the influence of Plomin and Daniels' (1987) article is that an entire field of empirical research was generated in an attempt to answer the question posed in its title: Why are children in the same family so different?

This was explained to the general public is a book titled, The Nurture Assumption. It argued against the assumption that parental nurturing is important.

The terminology is a little confusing. The twin studies divide influences into these factors:

genetic effects (heritability);
shared environment - events that happen to both twins, affecting them in the same way;
unshared, or unique, environment - events that occur to one twin but not another, or events that affect each twin in a different way.

So the "shared environment" is the family environment shared by siblings. Many people assume that this is the most important thing in a child's life, but the studies do not show any significance at all.

A recent blogger JayMan elaborates:

In my earlier post on Gregory Clark’s work, The Son Becomes The Father, I laid bare the case for the known high heritability of human behavioral traits (including values and attitudes) and life outcomes. As well, equally important, I illustrated the complete absence of shared environment influences on these – that is, the effect common environmental forces that children growing up together share. This includes parents and upbringing, making it abundantly clear that parents don’t leave a lasting impact on who we grow up to be. These are towards what I’m calling the “75-0-25 or something” rule,

These results are hard to accept, as nearly all parents believe that they are doing something special for their kids with their care.

My theory is that parenting does not seem to make much difference because most moms just do what all the other moms in the neighborhood do. Then kids seem to be influenced more by schools and peers, than by parents. Of course this means parents still have influence by choosing where to live, what schools for the kids to attend, and what friends are encouraged. Parents can have even more influence if they are willing to have policies different from the crowd.

Whether I am right or not, there is very little scientific evidence that any parenting style is any better or worse than any other. If someone tries to tell you that you should be doing something different, ask for the research. It will usually not exist.

Here is another article about exaggerated parental influences:

The study identified that although there is a lack of scientific foundation to many of the claims of 'brain-based' parenting, the idea that years 0-3 are neurologically critical is now repeated in policy documents and has been integrated into professional training for early-years workers.

Dr Jan Macvarish, a Research Fellow at Kent's Centre for Parenting Culture Studies, analyzed the policy literature for the study.

She said: 'What we found was that although the claims purporting to be based on neuroscience are very questionable, they are continually repeated in policy documents and are now integrated into the professional training of health visitors and other early years workers. "Brain claims" entered a policy environment which was already convinced that parents are to blame for numerous social problems, from poverty to mental illness.

'The idea that these entrenched problems will be solved by parents being more attentive to their children's brains is risible. Although aimed at strengthening the parent-child relationships, these kinds of policies risk undermining parents' self-confidence by suggesting that "science" rather than the parent knows best.'