You are right, this proves nothing. It would be interesting to see the paper, but I am not going to pay for it. And the graph they provided does not prove much

You are right, this proves nothing. It would be interesting to see the paper, but I am not going to pay for it. And the graph they provided does not prove much by itself. There are way too many assumptions to be made between the graph and the questionable conclusion. In any event I have a lot of suspicion that most of the discrepancy comes from trust fund and other inheritance income or nepotism. Adopted kids very rarely get any trustfunds. The test kids are in their 20s and 30s and usually at that age trustfunds and inheritances are contributed by people other than the parents (grandparents aunts uncles, etc.) and those people did not adopt the kids and usually do not have that much involvement with them. Furthermore, people that live large inheritances to their kids tend to be of the selfish gene type and those people rarely adopt. Rich people that do adopt tend to spread the wealth more to charities, etc. and do not give kids (whether adopted or not) huge inheritances. Large inheritances are rare but only a few of them could scew the data a lot. Similarly nepotism goes a long with gene selfishness. And will have similar effects to inheritances.