Yes, it is very hard. I know, because I know people attempting to find those genes. They report that it's very hard. I specifically said normal variation in IQ to make it clear that I'm not talking about mutations and variants which cause retardation. Those QTLs of large effect are easy to find, but they aren't implicated in normal human variation. How do we know this? Because they don't show up in linkage or association studies consistently.
I didn't say it's impossible. There were many things impossible 10 years ago that are possible now. But I didn't make that assertion in ignorance.
Here's an example of the kind of thing I mean:
"There is "a highly significant association" between the CHRM2 gene and intelligence according to a 2006 Dutch family study. The study concluded that there was an association between the CHRM2 gene on chromosome 7 and Performance IQ, as measured by the Wechsler Adult Intelligence Scale-Revised. The Dutch family study used a sample of 667 individuals from 304 families.[59] A similar association was found independently in the Minnesota Twin and Family Study (Comings et al. 2003) and by the Departme...
Link.
"Razib Khan has an academic background in the biological sciences, and has worked for many years in software. He is an Unz Foundation Junior Fellow. He lives in the western United States."
Razib's writings can be found on his blog, Gene Expression.