Probably the best known evidence supporting the idea that I.Q. scores are inherited comes from observations made on separated identical twins. That evidence seems especially powerful, because it is based on fewer-and simplerarbitrary assumptions than must be made to interpret other forms of data. There are two very different types of twins. When a single sperm fertilizes a single ovum, it sometimes happens that two different individuals develop. These individuals are identical, or monozygotic (MZ), twins. They presumably have identical genes. They are necessarily of the same sex, and their physical similarities are typically very striking. The other type of twin pair occurs when two different sperms fertilize two different ova at about the same time. The mother will then bear two different individuals, known as fraternal, or dizygotic (DZ), twins. Though born at the same time, these two individuals will be no more genetically alike than are ordinary brothers and sisters. They will have in common, on average, only half their genes. They may be same-sexed or opposite-sexed, and may be very dissimilar physically.