Seeing Double

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.

Take the Twin Test

Learn a bit about genetics and test your ability to discern between fraternal twins, identical twins and pairs of unrelated people.

  • Kyle and Keith
    While identical twins share almost the exact same DNA, there are plenty of things they don’t share. For instance, they have different fingerprints. Over time identical twins can also grow to look more and more different as the influence of the environment, as well as how their own genes express themselves differently.
  • Twins
    Not all identical twins look exactly alike and some fraternal twins look amazingly similar, so judging whether siblings are identical or not can sometimes be a challenge.
  • Heidi and Mary
    Only one-third of all twins in the world are identical. Although the likelihood of having fraternal twins is influenced by genetics , having identical twins is not.
  • Richard and Richard
    While twins often share similar personality traits, looking alike doesn’t necessarily mean that you share the same personality.
  • Amanda and Brenda
    Traits like intelligence are highly heritable, but genetics isn’t the only factor. Often times in twin studies much focus is placed on similarities, but the differences between identical twins can be just as telling.
    Several research studies have shown that the longer you spend with someone the more closely you tend to resemble them.
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