Discuss the difficulties in studying the genetics of human behavior traits

The search for genetic factors in alcoholism illustrates the problem of selecting the proper genetic model to analyze behavioral traits. Segregation and linkage studies indicate that there is no single gene responsible for most cases of alcoholism. If a multifactorial model involving a number of geneseach with a small additive effectis invoked, the problem becomes more complicated. How do you prove or disprove that a specific gene contributes, say, 1 to the behavioral phenotype? At present, the only method to accomplish this would involve studying thousands of individuals to find such effects, and at present, few large-scale GWAS have been completed.