Nature Vs. Nurture

Today most interest in twins serves to determine whether traits come from ones environment or from genetics. However, throughout time, twins have been a constant source of entertainment and amazement for society. Twins differ very little, and according to an e-mail interview with Dr. Nancy Segal, the American attraction to them is due to our high value and appreciation of independency and personal differences. Segal believes the similarity of twins contrasts greatly with Americans values and this contrast sparks interest, almost to the nature of a circus sideshow. Because most singletons find the eerie alikeness of twins mystifying, myths and urban legends about twin behavior are not uncommon. For instance, the belief twins share more than just a special bond, but a type of telepathic connection, is commonly held. To explain this seeming telepathy, Piontelli writes:

It would not be unreasonable to hypothesize that an uncommonly early and constant exposure to each other's rhythms, bodily substances, behavioral patterns, and body language may foster a heightened reciprocal sensitivity in some twins. Some kind of familiarity with each others rhythms may already start before birth. This would not imply esoteric ‘telepathic understanding'. It may only suggest that twins, being constantly exposed to the manifestations of their co-twins, could sense from an unusually early age, and hence later perceive in an uncommon way, the signals of the other twin. (120)

So twins do not have extrasensory perception, but then, why are they so similar? Is it because they are raised in the same environment or is it simply because they share the same genes? Twins may be mystifying to most the general population, but they are very interesting to scientists studying genetics. Today twin study is the major source of information fueling the scientific debate of nature verses nurture. Nature refers to traits genetic or inherited from parents and nurture refers to all traits non-genetic or stemming from the environment. The answers received from this debate help to define human differences. Understanding these differences can eventually prevent and help in the early treatment of disease. It may even be able to predict diseases in unborn children that would not show signs or symptoms for decades. Scientists cite these and other interesting reasons as impetus for studying twins. However, the majority of twin studies serve to determine whether genetics or environment decide human traits.

Studies of twins place them in two differing groups. Identical twins or monozygotic twins, according to Clegg and Woollett, "are formed by the accidental splitting of a single fertilized egg, so that the babies develop from one egg and one sperm." They also define fraternal or dizygotic twins as non-identical twins that "develop from the fertilization of two eggs by two sperm" (16). It is common knowledge ones genes are inherited, half from ones mother, and half from ones father. Since monozygotic, or MZ, twins are a split of the same egg and sperm, they contain identical genes. However, the genes of dizygotic, or DZ, twins are only as similar as typical brothers and sisters. Scientists are interested in fraternal twins reared together because, being the same age and in the same home, they share practically identical environments. They are also interested in studying identical twins that have been reared apart from one another, because they share identical genetics but have differing backgrounds. These groups of twins provide researchers with automatic control groups.

By studying these two groups, scientists can see if DZ twins with the same environment are more similar or if MZ twins with the same genes are more similar. Rowe explains the importance of these studies:

The study of separated twins is the most direct method for estimating a trait’s heritability. Separated MZ twins are reared by different parents, and hence have different family environments, whereas they possess identical heredity. Thus the effects of heredity are distributed against a background of different family environments, and we can infer genetic influence on a trait from resemblance in separated twins (37).

Many scientists who study twins reared apart believe traits are mostly genetic and they seek to find a genetic predetermination for