I doubt that much scientific was done to asses those sorts of things.
But one thing science did asses was IQ comparison of identical twins (monozygotic - from the same egg).
Interestingly it turns out that IQ correlates most strongly with twins so long as they are well fed and taken care of. However, if you don't feed a child or don't provide a healthy environment things don't go as well.
The initial 1990 study was very controversial - so much so that it was not funded via "traditional" means. And, according to this WSJ article, it received a great deal of criticism. Subsequent studies, however, have shown that the initial study, conducted by Thomas Bouchard and Nancy Segal at the University of Minnesota, was quite accurate.
The bottom line is that identical (monozygotic) twins raised apart in similarly "healthy" and "well fed" environments have similar IQ's around 70% of the time. Take away "healthy" or "well fed" and this falls off dramatically.
However, identical twins raised together had a 88% chance of similar IQs.
If you start looking at fraternal twins, siblings, cousins, and so on the number drop off rapidly.
So all discussions of preferences for mates and cigarettes, what does this say?
I think that the interesting numbers are not the number of similar IQs but instead the numbers of dissimilar IQs.
For example, if 70% of identical twins raised apart have similar IQs, what about the 30% that don't?
More interesting, what about the 12% (opposite the 88% with similar IQs raised together) that don't?
So you have identical genetics to your identical twin sibling but you don't share a similar IQ?
It would seem the first thing this says is regardless of genetics your personal results with IQ and brain development may vary significantly.
Why could this be?
For one thing, the structure of your brain is determined in part by the stimulation you receive as a young child, say under the age to two or three. So at least theoretically one twin could be "favored" in some way over another during childhood causing the other to receive less stimulation.
On the other hand, one twin my find their environment more compatible with there constitution than the other creating a situation where one grows more than the other.
Yet even identical twins do not have the same fingerprints.
Our brains on average (according to Wikipedia) have some 10^14 synapses - as children we have some 10^15.
As we grow our brains change and we lose connections (or the numbers stabilize).
Synapses are not simple logic gates like you might find in a computer. They are influenced by chemicals in the brain as well as though electrical activity.
And no one really has any good idea about how, for example, our memory's actually work in terms of neural function. Or how we process audio or visual data.
The bottom line, of course, is that its impossible to ensure that each child receives exactly what's required because even genetically identical child raised in the same household 12% of the time have different IQs.
In fact, we don't even know how to guarantee the same outcome if we tried. (One would expect on average that identical twins were raised more or less identically, i.e., one was not locked in the closet.)
It seems to me like this is a case of Wolfram's "Computational Irreducibility" in the sense that we have no way to work backwards from a given set of twins and their IQs to see how things got that way.
It also seems, based on the fact that finger prints of identical twins are not identical, that parts of a of human beings physical structure vary even given identical genetics, i.e., are their parts of your brain that are like your fingerprints?
Some science has recently revealed that even identical twins are not identical at other levels: see this. Which is interesting... Perhaps genetics is not as "strong" as we though in its influence over how we grow.
So I guess at even the most basic level we are always going to be different...
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