Almost no one wants to be called a "eugenicist." It's a term of abuse. But if you go back to the origin of the term, it basically amounts to the following two claims:
Claim #1: One of the main causes - if not the main cause - of economic, cultural, and other forms of success is genetic.
Claim #2: Policy-makers can make their societies more successful by improving the quality of their societies' genes. For instance, the famous eugenicist Karl Pearson maintained that Britain should only admit immigrants who "raised the average":
What is definitely clear, however, is that our own Jewish boys do not form from the standpoint of intelligence a group markedly superior to our natives. But that is the sole condition under which we are prepared to admit that immigration should be allowed.
These days, there is massive empirical support for Claim #1. For primers, see here and here. The result is that people who fear Pearson-like policies engage in a lot of silly ad hominem attacks on defenders of Claim #1. And on the other hand, some defenders of Claim #1 are happy to follow in Pearson's footsteps by advocating policies inspired by Claim #2.
The problem, however, is that Claim #2 simply does not follow from Claim #1. Even if genetics explained ALL differences in success, many policies that raise average genetic quality would backfire. How? Let me begin with a thought experiment, then explain the general principle.
Suppose we have an isolated society in which everyone is a genius. Let's call them the Brains. Who takes out the garbage? A Brain, obviously. Who does the farming? Again, Brains.
Now what happens if the geniuses come into contact with a society where everyone is of average intelligence at best? Let's call them the Brawns. If the Brains allow the Brawns to join their society, the average genetic quality of the Brains' society plummets. But everyone is better off as a result! Now the Brains can specialize in jobs that require high intelligence, and the Brawns can take over the menial labor. Total production goes up.
This is an example of what economists call the Law of Comparative Advantage. Trade between two people or groups increases total production even if one person or group is worse at everything. Suppose, for example, that Brains can make 5 Computer Programs or 10 Bushels of Wheat per day, and Brawns can make .1 Computer Programs or 5 Bushels of Wheat per day.
Bushets of Wheat
Brains and Brawns can still trade to mutual benefit: Just have one Brain switch from farming to programming (+5 Programs, -10 Bushels of Wheat), and three Brawns switch from programming to farming (-.3 Programs, +15 Bushels of Wheat), and total production rises by 4.7 Programs and 5 Bushels of Wheat.
My colleague David Levy and his co-author Sandra Peart have taken Karl Pearson to task for his laughable claims about the ability of Jewish immigrants. But the deeper lesson is that even if Pearson's judgment on this point were correct, his policy recommendation would be counter-productive on his own terms. Yes, admitting geniuses leads to greater achievement; but admitting non-geniuses achieves the same effect, by encouraging native citizens to switch to brainier work.
Of course, this doesn't mean that improving the quality of your societies' genes never leads to greater success. Holding the number of people constant, more quality leads to more results. But an increase in the number of non-geniuses, holding the number of geniuses constant, will also normally cause higher levels of achievement through specialization and trade.
Thus, there is no reason for opponents of Pearson-like policies to pretend that twin and adoption studies don't exist, or "don't prove anything." The Law of Comparative Advantage shows that even if some people really are more productive than others in every respect, they have something to offer each other.