Monozygotic twins (left) share 100% of their genes in common, whereas dizygotic twins (right) share 50% of their genes, the same as other siblings. Studies of monozygotic and dizygotic twins help researchers estimate the relative contributions of genes and environmental influences on behavior.
PAUL AVIS/GETTY IMAGES
JBPHOTO1/ALAMY
JBPHOTO1/ALAMY