The researchers studied the heritability of racing performance in Thoroughbred horses. They found that the heritability estimate was 0.55, meaning that genes play a significant role in racing ability. A selection index was developed to choose young sires based on their potential to produce successful offspring. The index accurately predicted the average earnings of 67% of the horses in the study. The most important factor in determining a sire's breeding potential was found to be their own racing performance.