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 bedrock for much of what we know about the genetics of many traits, diseases and other conditions.
For scientists, twins offer a ready-made test of nature and nurture and their paired roles influencing everything from height, to disease risk, to intelligence. And 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 the their similarities than their differences, but the differences can often be just as illuminating. Last year, a meta-analysis looking at more than 50-years of twin research –which included studies on more than 14.5 million pairs of twins – estimated 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 twin 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.