Sometimes rare experimental outcomes tell you a lot, as when a few of the alpha particles impinging on a gold foil were scattered straight back [Geiger-Marsden experiment] , implying that most of the mass and positive charge of an atom is concentrated in a tiny nucleus.
Let’s think about the Jim Twins. Chevys, Miller Lite, migraines, chain-smoking, finger-nail chewing, baseball hating, vacationing on the same three-block beach near St, Petersburg, both leaving love notes around the house, both building a circular white bench around a tree in the front yard. Tests of intelligence, personality, heart rate, and brain waves were about as similar as the same person taking the test twice.
Let’s face it: the Jim Twins were the same guy twice. At least in this case, Nature utterly pwned Nurture.
Sure, neither could have driven a Chevy in 1500, or had a Miller Lite – so the environment influenced them all right, but it influenced them in exactly the same way. Your preferences can be your constraints. They were exposed to similar environments ( raised in similar small towns in Ohio) – but there are always differences. Different adoptive parents, different schools, different teachers, different friends – but none of those differences seem to have had any lasting consequences.
If many small influences kept nudging them, differences should have grown with time, like a random walk. Didn’t happen. Along the same line, heritability of intelligence increases with age: people converge onto the predicted phenotype. Again, little differences don’t accumulate.
In order for that to be the case – the development process has to be canalized. Most environmental differences must end up having no effect – they don’t generate differences between twins. Let me try an analogy. Think of a river basin: a drop of rain that falls into the Rio Grande basin is going to end up in the Caribbean, whether it lands in Taos or Albuquerque or El Paso. It matters which basin a drop lands ( Rio Grande vs Colorado), but it doesn’t matter where in that basin lands.
What can knock you into a different basin? Make you a different person? Well, trauma can: closed head injury, firing a steel rod through your head (Phineas Gage), serious illness ( Helen Keller), shortage of a vital nutrient ( iodine and cretins). However those aren’t the environmental influences people want to exist: they want going to school X instead of school Y to make a difference (in your capabilities or personality: X and Y can certainly put different ideas in your head, for instance if X=More Science and Y = Commie Martyrs). Or, winning the lottery ought to make people different (in personality and capabilities) – but it doesn’t. Not in typical modern circumstances, anyhow: in medieval times t might well have kept you from starving to death.
The environment influences people want to exist, mostly don’t.
In order to push people into a different basin of attraction, make them a different person, you apparently need to to thump them pretty hard. In the cases we understand, that means damage.
Usually when a guy is homosexual, his identical twin is not. Family environment doesn’t look to be the cause: something thumped the homosexual twin.