An interesting article on BBC News titled “What made Darwin a creative thinker” which comments on how instinctive behaviour can be attributed on having older brothers and sisters.
A study of 120,000 people concluded that the instinctive behaviour adopted by younger children to help get them noticed can stay with them for life.
More at the BBC science site ::: http://news.bbc.co.uk/2/hi/health/2400447.stm