Twins have long fascinated us as humans, so much so that twin stories are ingrained in the mythology of almost every culture.
Beyond folklore, the study of twins has also become the bedrock for much of what we know about the genetics of traits, diseases, and other conditions.
For scientists, twins offer a ready-made test of nature and nurture, with their paired roles influencing everything from height to disease risk to intelligence. Looking at the similarities and differences shared between fraternal twins and identical twins has brought into sharp relief some of what can be explained through genetics and what can be explained from the environment.
When we look at identical twins, we often are drawn more to their similarities than their differences, but the differences can often be just as illuminating.
Last year, a meta-analysis of more than 50 years of twin research—which included studies on more than 14.5 million pairs of twins—estimated that the influence of genes and the environment across all traits is about equal.
Twins can be identical (monozygotic) or fraternal (dizygotic), but there are several other different types, including half-identical, mirror twins, and fraternal twins from different fathers (it happens). The rate of twin births has actually increased in the last 30 years, going from about 2 percent of all births to more than 3 percent.
Some of this may have to do with fertility treatment, and some might be related to women having children later in life. Older women are more likely to have twins than younger women.