(from Twins and Supertwins) Twin studies began only in the latter quarter of the 19th century, and previous knowledge about twins had been based on folklore and myth. "Notice had been taken of the different types of twins" but other than outright observation of birth instances, little was known as to the hows and whys of twinning. In 1875 Dr. Francis Galton, a British scientist, made the first guess that look-alike twins were alike in their hereditary make-up, and may have come from a single egg. He also surmised that look-different twins might have come from two separate eggs. Though there was no evidence to support his guess, Dr. Galton suggested that a study of the two types of twins might shed some light on the influences of heredity and environment. More studies of heredity followed, which provided additional findings on human inheritance. Twins are still used a resource from which to determine the influences of "nature" and "nurture." Studies involving twins and their parents follow these general guidelines and patterns:
1)Pre-natal and birth experiences of twins--stats and info about twin conceptions, prenatal development and delivery.
2)Trait-similarity studies--explorations on the extent of similarity in given traits (physical, psychological, biochemical) appear in identical and fraternal pairs. Identicals share hereditary information, so these studies have illuminated how far certain traits are or are not influenced by heredity.
3)Longitudnal studies of twins--continuing studies on pairs of twins for extended periods of time. Comparisons of growth rates, health, intelligence to the rates of singletons.
4)Comparison studies of twins and singletons--study of traits in twins as compared to those in singletons, to find out whether these traits occur more often in twins or singletons. Studies have revealed some drawbacks to twinship, such as social development, language acquisition and health problems.
5)Psychological relationships between twins--investigations into the effects of rivalry and jealousy between twins; dominance of one twin over the other; separating or keeping twins together; birth order; degrees of attachment between twins.
6)Co-twin control studies--Deliberate training or conditioning of two members of an identical twin pair in specific ways for a determined time period, then seeing what differences develop between them, and how long they last. Such studies are made in respect to behavioral conditioning, or the effects of diet, exercise, or given treatments.
7)Monozygotic twins reared apart (MZA)--findings from MZ twins reared apart.