A significant portion of the book is based on statistical correlation. The book makes multiple references to Mankind Quarterly.
The issue is not whether science can or should study this. It is the dangers of doing so using bad science then packaging up unsupported results and presenting them in a way that justifies harmful division in society on a foundation built of sand.
If it were serious science, it would surely have looked beyond Caucasian Americans and investigated the intelligence of Asian Americans too.