Athletic performance by horses is a heritable trait. This seems an obvious statement given that decades, and for some breeds centuries, of breeding have been aimed at selecting and breeding horses with superior performance based on the assumption that athletic capacity is heritable. Recent studies in Arabian, Thoroughbred, and Quarter horses have demonstrated the heritability of best racing time, race performance (as assessed by finishing position), and earnings. However, as with any trait that is likely to be polygenetic and not controlled by a single gene, the heritability of performance characteristics is not particularly strong, reflecting the effect of environmental factors, including factors as early in life as the intrauterine environment, on future athletic capacity. There is considerable interest in identifying genes associated with performance in human athletes, and in identifying those variants (alleles) that are associated with either superior or inferior performance. The number of genes associated with performance in human athletes continues to increase each year, and is cataloged in an annual update. The 2005 update listed 140 genes associated with performance or health-related fitness in humans. Enhancing sport performance is part of the discipline of psychology known as sport psychology. Sport psychology is the scientific study of people in sport and exercise contexts. There are two main objectives of sport psychology: (a) learning how psychological factors affect an individual's physical performance, and (b) understanding how participation in sport affects a person's psychological development, health, and well-being.