Final answer:
Population stratification is the cause of false positive associations that arise from the use of mixed populations in an association study, as it refers to allele frequency differences between subpopulations
Step-by-step explanation:
A false positive association in an association study due to the use of a mixed population is most commonly caused by population stratification. This occurs when variations in allele frequencies between subpopulations, which could be due to ethnic, geographical, or social structures, lead to spurious associations with the trait of interest. It's different from genetic drift, which is a stochastic process that affects allele frequencies in a population over time. Genetic drift can cause allele frequencies to change at random, which may lead to differences in genetic variations between populations, but it does not specifically result in the false positives as described in the question. Gene flow and linkage disequilibrium are other genetic concepts, but when considering false positive associations due to mixed populations, population stratification is the key factor.