People in the learned/genetic flamewar - let’s separate out two things:
- Personality (MBTI, or similar, sets of behavioural preferences)
- Behaviour (a function of personality and context.)
Personality has both genetic and environmental components, and tends only to “settle down” when someone is about 7 (I don’t have a reference for this, but I did just hear a lecture on it, and I trust the academic who gave it, at least on this point.) In any case, we can probably assert that there’s a strong influence from genetics, especially outside of “extreme” environments.
Behaviour is a function of personality, but it is also strongly influenced by context, and the context difference between single and multi-child households is huge.
I would argue that parental (and indeed sibling) behaviour, is a strong driver of the child’s behaviour; I’d be interested in studies that seek to find ways to separate out the genetic component (although they’d be difficult to implement owing to ethical rules; treating two twins differently, for instance, would probably be unacceptable, and unless they were isolated from each other there would be behavioural cross-pollination anyway!)