Two genome-wide association studies sought genes that contribute to intelligence. The first considered 1000 Mexican-American participants in the San Antonio Family Heart study, and found evidence pointing to genes on chromosomes 3,6 and 18. The second study looked at 1,094 children with attention deficit hyperctivity disorder and their siblings, and found gene candates for intelligence on chromosomes 7, 9 and 14. Suggest two possible reasons why the two studies had different results.