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.
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.
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.
Conception of twins can happen on different days. However, the two conceptions must take place in hours or a few days within each other. This can happen, for example, with a set of fraternal twins through the conception of two separate eggs and two separate twins.
The odds of having a second set of identical twins are extremely low. However, the chances of a woman having fraternal twins after having fraternal twins is three to four times higher than the rest of the population. And if you are using fertility treatment, then the chances of a repeat are high as well.
A high rate of twinning has been found in Nigeria. Some cultures in this part of the world consume an abundance of yams, which contain phytoestrogens. A diet high in this nutrient might affect the occurrence of twinning.
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.
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.
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.
Yes. An analysis of 19 different studies on left-handedness in twins showed that 15 percent of identical twins and 12 percent of non-identical twins were left-handers. The overall frequency of left-handedness in the general population is about 9 percent. Twins indeed have a slightly higher chance of being left-handed than the rest of us.
Twins start interacting with each other as early as three and a half months in gestation. Such interaction has been caught on MRI scanning, giving the appearance of twins fighting for space.
This phenomenon is called a vanishing twin, which is considered a miscarriage. One twin or multiple dies and is absorbed by the other multiple, the mother, or the placenta. This can happen in up to a third of twin or multiple pregnancies.
These siblings are not, in fact, twins; they are born within 12 months of each other. This term came about in the late nineteenth century, when large Irish families immigrated to the US. The phrase Irish Twins was considered disparaging.