For the same reason that evolution, global warming, the historical study of religion, etc. are controversial: it conflicts with popular political beliefs and leads to taboo discussions.

It's taboo to both the left and the right. Both like to pretend differences in innate ability don't exist. The left likes to explain disadvantage exclusively as a socially constructed phenomenon that can be remedied through social engineering. The right likes to chalk it up to laziness or moral corruption, arguing that the disadvantage more or less deserve it.

If it's biological to any significant degree, they're both wrong. It's heritable, so no amount of social engineering will fix it, and it's also not a result of chosen moral deficiencies or laziness so it's not a moral failing. Being born with a lower IQ is no more your fault than being born without an arm.

It also leads to taboo discussions around wealth distribution. If our success rates are largely a result of our success at winning the genetic lottery, can you morally justify really massive levels of wealth inequality? Is it just that random initial conditions completely dictate outcome?

If intellectual ability is largely heritable, then both the left's PC moralizing and the right's Calvinist moralizing are bullshit.

I've toyed around with the concept of a kind of transhumanist socialism in which these kinds of enhancements -- both germ-line and for already living people -- would be made universally available to everyone. I wonder to what extent the failure of socialist schemes is the result of their technical inability (or unwillingness) to address the root causes of inequality. Use a progressive income tax not to fund the zero-sum redistribution of wealth, but the positive-sum redistribution of ability. Transform humanity into a "runaway self-improving AI."

As far as I know, virtually everyone in every political persuasion or "wing" would be horrified by that concept. Given what garbage pretty much all of today's political discourse is, it kinda makes me think I'm onto something.

For the same reason that it's still controversial that chimps go to war even with not threatened. Some scientists see findings like this as an affront to their closely-held beliefs about humans and human nature.

And since these beliefs often have political implications, human tribalism takes over. What begins as a benign debate over, e.g. the validity of research methods, often devolves into controversy that's actually a proxy war for some tangentially related idea.

>How is this stuff still controversial?

Because people are scared to make claims about trait heritability in humans. The moment someone publicly says anything about e.g. genetics influencing human intelligence, someone flies out of left field and accuses them of being a racist.

The absolute inanity of political correctness has turned certain areas of scientific inquiry into political minefields.

Because, to begin with, the idea that we can measure intelligence is still under debate. There are people on HN who will tell you it's not, which is only true if they choose to ignore everyone who disagrees with them. And because we know how the media likes to spin/exaggerate medial studies, someone one will eventually link it to race. And it's happened before[1]... . And, similar to how BMI wrongly turned into an individual indicator of obesity, some very mild affinity over a large sample size for a dubious metric will turn into the absolute brain-points and politics will take over and it'll just be a big mess.

In short, society is not mature enough to handle this area of research right now. The consequences of getting it wrong, or even right, is severe.

1. http://en.wikipedia.org/wiki/Scientific_racism

Leads the conversation to many taboo topics around intelligence, e.g. racial differences.

Because it is not actually science.