How Twins Contribute to the Debate
Twins come in two varieties: fraternal and identical. Fraternal twins arise when two separate eggs are fertilized by two different sperm simultaneously. According to Mendel’s Law of Segregation and Law of Independent Assortment, no two eggs or sperm are completely alike in genetic makeup; therefore, fraternal twins are just as likely to share the same amount of genetic material as siblings born from different pregnancies--about 50%--but just happen to share a womb.
On the other hand, identical twins arise from one egg fertilized by one sperm. During embryogenesis, the single fertilized zygote splits into two zygotes, and both continue forming embryos. Identical twins share 100% of their genetic material because each resulted from the same egg-sperm union.
Twins are natural experimental models, because each set of comes with its own control. Identical twins share 100% of their genetic material, i.e. they have the same nature, so any differences between the two in personality can be attributed to nurture. This is especially evident when identical twins are separated early on in life and raised in different environments. The situation gets more complicated when identical twins are raised in the same environment, yet have different personalities.
Fraternal twins share roughly 50% of their genetic material. If they are raised in the same environment, any differences in their personalities can be attributed to differences in their genes. However, fraternal twin studies are best carried out when they are raised in the same environment, so as not to introduce more variables.