Scholars have long studied twins to address the naturenurture
question. Identical twins separated at birth and
brought up in separate environments provide the ideal test
cases, but such cases are exceedingly difficult to find.
Instead, most researchers study twins who have grown up
together and carefully compare the experiences of identical
twins to those of fraternal twins. Since identical twins are
monozygotic, developed from a single sperm fertilizing a
single egg, which then divides into two separate cell masses
within the first two weeks of development, these twins
are essentially clones. Fraternal twins, on the other hand,
are dizygotic, developed when two eggs are each separately
fertilized by different sperm. Identical twins like Dear
Abby and Ann Landers have all their genes in common