Home > Encyclopedia of Psychology > Twins

Two children or animals born at the same birth.

Identical, or monozygotic, twins are of the same sex and are genetically and physically similar because they both come from one ovum, which, after fertilization, divides in two and develops into two separate individuals. Fraternal, or dizygotic, twins occur when the mother produces two eggs in one monthly cycle and both eggs are fertilized. The conceptions may take place on two separate occasions and could involve different fathers. Fraternal twins, who are no more genetically alike than ordinary siblings, may be of the same or different sex and may bear some similarity of appearance. Twin pregnancies occur on the average in one out of every 80 to 100 births. However, the incidence of twins reflects the number of twin babies born per thousand completed pregnancies, and it is a fact that many more twins are conceived than are born.

The causes of...