Studying identical twins raised apart is another classic strategy to distinguish environmental and genetic factors. Identical twins share identical genetic material, and fraternal twins share 25 percent of their genes. In 1991, Dr. Stunkard examined sets of fraternal and identical twins in both conditions of being reared apart and reared together.4 Comparison of their weights would determine the effect of the different environments. The results sent a shockwave through the obesity-research community. Approximately 70 percent of the variance in obesity is familial. Seventy percent.
