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The special relationship between twins allows researchers to examine the differences between genetic and environmental influences over both physical and mental health, as well as traits and behaviors. By studying twins, we can learn a lot about diseases, disorders, and human nature in general. Research on twins helps answer questions about many aspects of being human. About three or four in every 1000 births are identical twins.

There are different types of twins, identical and fraternal being the most common. Identical, or monozygotic, twins develop from the same fertilized egg that splits into two. These twins are of the same sex and share the same genes. Sometimes called paternal twins, identical twins provide evidence of a high degree of genetic influence on human development. For fraternal or dizygotic twins, they develop from two different eggs fertilized by separate sperm; they generally share about half of their genes. Therefore, fraternal twins are essentially like typical brothers and sisters.

For more on identicals and fraternals, visit Identical Twins and Fraternal Twins.

The Science of Twins

You may wonder which parent controls the predisposition, if any, to have twins. Is it mom or dad? A mother’s genes matter in the case of twins simply because only women ovulate and men don’t. A woman may have the genetic tendency to hyper-ovulate or release more than one egg during a cycle. Plus, genes have a hand in fraternal twins and they do run in families. A father can be, however, a silent carrier and pass on the tendency for hyper-ovulation to a daughter. However, genetics do not influence the birth of identical twins, having identical twins is random and they do not run in families.

For more on the science of twinship, visit The Lives and Relationships of Twins.

Do older moms have more twins?

Yes. Older mothers, in their 30s and their 40s, have more estrogen than younger moms, and therefore they can produce more eggs at a time. Plus, the more pregnancies a woman has had her chances of producing twins increases.

Can twins result from fertility treatment?

Treatments such as fertility drugs or in-vitro fertilization can heighten the chances of having fraternal twins because of the increase in both eggs and sperm in the uterus.

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The Quirks of Twinship
Ryan McGuire Gratisography Shutterstock

The fascination with twins is evident. The Internet, for example, is filled with questions on the peculiarities of twinship. There are queries about twinship skipping generations, whether more twins are left-handed, why they suffer separation anxiety, if there is fighting inside the womb, among other curiosities.

Twins are also more commonly female, rather than male. But male embryos die at a higher rate while in-utero than do female embryos. The death rate is also higher for twins in-utero overall.

For more on the child development of twins, visit How to Raise Twins.

Do twins skip generations?

There is no empirical proof that twins skip generations. However, if the father is a silent carrier of the gene for hyperovulation, he may pass the tendency for twins on to a daughter. This gives the appearance of “skipping” a generation.

Why do some twins look like mirror-images of each other?

This can happen when the female egg splits later, in about nine to 12 days after conception. Because of this delay, these twins look like mirror images of each other; for example, birthmarks appear on opposite sides of their faces, or one twin may be right-handed and the other twin left-handed.

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