Outcrossing is the most common form of sexual reproduction among animals and plants. Despite this prevalence, the selective pressures maintaining the widespread use of outcrossing are not fully understood. Evolutionary theory predicts that outcrossing populations are less susceptible to the fixation of deleterious mutations and may more rapidly adapt to changing ecological conditions than self-fertilizing populations. We empirically tested these predictions using experimental evolution, by exposing obligate selfing, mixed mating, and obligate outcrossing populations of C. elegans to elevated mutation rates and rearing them in a rugged novel environment. After fifty generations of mutation and selection, outcrossing populations fixed significantly fewer deleterious mutations than selfing populations and exhibited significantly greater rates of adaptation under natural mutation rates. To our knowledge, this work is the first empirical test of theory regarding the evolution and maintenance of outcrossing and demonstrates that, as predicted, outcrossing is conditionally favored over self-fertilization. However, the conditional value of outcrossing may be quite significant as most organisms are presumably subject to deleterious mutations and/or environmental conditions favoring rapid adaptation. Both of these factors likely explain the prevalence of outcrossing as a sexual mating strategy.