The question when we see exceptions, whether it's the Lykken and Tellegen study, not all twins were the same, the question is no longer whether or not change is possible, but rather how is change possible.
If the first six months of a child's life matter as much as most people think they do, then spending even that short time together could influence the result of a twinsstudy.
Maybe Galton's classic twinsstudy was invalid: perhaps you cannot look at the similarities between identical twins, and those between non-identical ones, and conclude that the difference must necessarily be due to genes.
