The argument is not that a peer group affects all members in the same way, but that the effect of the peer group is dominant.

Just responding here as well. The book actually puts forward genetics as the most important cause of behavioral differences. It then argues that the next most important cause is not parenting but what is referred to in the literature as non-shared environment (aka, everything other than parents, family, neighborhood, etc, which are all grouped under shared environment). Shared environment has zero effect.

It's all inter-related. The peer groups matter, but how they matter varies based on the environment.

As one example, you may have a kid who has a shitty family life who will act up to demand attention. If you have inexperienced teachers in a classroom of 35, it's going to be difficult to negate the impact of that kid on the classroom.

There's actually survey data about this. They ask people, basically, 'How much do differences in parenting style and decisions affect a child's personality/behavior?"

Everyone in the survey thought there was a strong effect except one group: Mothers with two or more kids.

There's probably some bias here where no parent is going to want to admit that they've treated their kids differently (likely unintentionally). Not to say that there isn't useful insight here anyway.