Not to disagree, but merely to raise a point: the distinction is being made between heredity and environment. But the implication is the environment is exactly the same for first born and later siblings. I’m not certain we can “prove” that. In our house we tried to spend the same effort with each child, but I’m almost certain we were more involved with our first. Everything was new, all excited, etc. Does that “natural” inclination lead to differing personality traits, and ultimately to higher IQ? Don’t know, just asking.