According to the NIH: Researchers have conducted many studies to look for genes that influence intelligence. Many of these studies have focused on similarities

According to the NIH: Researchers have conducted many studies to look for genes that influence intelligence. Many of these studies have focused on similarities and differences in IQ within families, particularly looking at adopted children and twins. These studies suggest that genetic factors underlie about 50 percent of the difference in intelligence among individuals. https://ghr.nlm.nih.gov/primer/traits/intelligence https://ghr.nlm.nih.gov/primer/traits/intelligence So, going with the NIH, I'd say about half.