Most higher organisms reproduce sexually, despite the automatic reproductive advantage experienced by asexual variants. This implies the operation of selective forces that confer an advantage to sexuality and genetic recombination, at either the population or individual level. The effect of sex and recombination in breaking down negative correlations between favorable variants at different genetic loci, which increases the efficiency of natural selection, is likely to be a major factor favoring their evolution and maintenance. Various processes that can cause such an effect have been studied theoretically. It has, however, so far proved hard to discriminate among them empirically.