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Personality Development in Twins

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Personality Lab:

Proseminar in Development:

Personality Development in Twins

I am a fraternal twin. I have, for the longest time wondered about how me and my brother think in relation to eachother. Why did Adam turn out to be a math genious and I became the writer? Adam and I both have the same neck lines given, but what of Adam’s hazel eyes? Mine are brown and so are my mother’s and father’s. I have studied the “nature versus nurture” debates well last semester in General Psychology, and in my AP Psychology course in high school. Still, I wonder why my brother and I are such polar opposites. We are twins after all and we have lived together for our entire lives.

Until college I had never been away from Adam for more than a day, and even now he is only a couple of hours north at Rochester Institute of Technology. We shared the same friends, the same computer, and the same room the same toys, the same video games, the same car, even almost all the same classes in high school; all these shared for almost nineteen years. It intrigues me how even though we had very similar experiences growing up we are still so opposite each other. He is quiet, has freckles, the hazel eyes, and a round face. I have an oval face, brown eyes and I love meeting new people. What makes us so different? It’s a mix of genes and our environments right? The evidence must run deeper than that.

Twins are fascinating. Beyond the statistical likelihood, or lack thereof, there's something downright captivating about twins. They can be boys, girls, one boy and one girl; they can be natural clones of each other, or totally different. What makes twins tick? How do we explain why one twin, identical or otherwise may grow up to be a rocket scientist and the other a poet? One with a flare for the night life and the other more solitary? Or how about why some twins seem identical in almost every way? How do twins differ along the lifespan? The answers may lie in both genetics and the environment in which they live, but it is not so clear cut as one or the other.

There are several types of twins one of which is the fraternal twin set. Faternal twins are born when two fertilized eggs are implanted in the uterine wall at the same time. The two eggs form two zygotes, and these twins are therefore also known as dizygotic twins. Dizygotic twins, like any other siblings, have an extremely small chance of having the exact same set of chromosomes. Like any other siblings, fraternal twins may look very similar, particularly given that they are the same age. However, fraternal twins may also look very different from each other and may be different sexes or the same sex. The same holds true for brothers and sisters from the same parents, meaning that fraternal twins are simply both brothers, sisters or one brother and one sister who happened to be born at the same time (Hall, 1996). Studies show that there is a genetic basis for fraternal twinning. However, research suggests that it is only the “…[female] partner that has influence on the chances of having fraternal twins as the number of eggs a woman produces is [unaffected] by the male partner” (Piontelli, 2002).

Identical twins occur when a single egg is fertilized to form one zygote (termed monozygotic) which then divides into two separate embryos. The two embryos share the exact same DNA; however, this does not mean that two exactly alike children are created (Hall, 1996). Even subtle differences in the children’s’ lives lead to major differences between the two. Personalities also generally develop differently. This is not considered to be a hereditary trait, but rather an anomaly that occurs in birthing at a rate of about 3 in every 1000 deliveries worldwide (Piontelli, 2002). If the zygote splits very early (in the first 2 days after fertilization) they may develop separate placentas and separate amnionic sacs. These are called dichorionic, diamniotic twins. In most identical twin cases the zygote splits after 2 days, resulting in a shared placenta, but two separate sacs. These are called monochorionic, diamniotic twins. In about 1% of identical twins the splitting occurs late enough to result in both a shared placenta and a shared sac called monochorionic, monoamniotic twins.

One other combination exists where the zygote “…may split extremely late, resulting in conjoined twins” (Hall, 1996). Of all twins those born conjoined have the highest mortality rate due to complications involving either shared organs or complications during surgery to separate the twins from one another. Conjoined twins have a very high in-utero mortality rate, about 60%. This is often due to umbilical cord entanglement prior the point in gestation where the twins may be removed from the uterus and have a fair chance of survival (approximately 32 weeks) (Bocklage, 1990).

Monochorionic, diamniotic twins have about a 25% mortality due to a complication called twin-twin transfusion (Bocklage, 1990). Twin-twin transfusion is the result of an intrauterine blood transfusion from one twin to another twin; basically when blood passes disproportionately between twins who share a placenta (Senat, M.V., Deprest, J., Boulvain M., Paupe, A., Winer, N., Ville, Y., 2004) (See Figure 1). The donor twin is smaller and anemic at birth and the recipient twin is usually larger and looks flushed at birth.

Dichorionic, diamniotic twins have the lowest mortality risk at about nine percent (Bocklage, 1990). This is most likely due to miscarriages during mid-pregnancy because the increase in size of each baby quickly leaves little room for the other. Malnutrition for both mother and children is another reason for unsuccessful births. Still, the percentage is low and even if Dichorionic, diamniotic twins are delivered early they generally are removed close enough to the end of the gestation period that they may be cared for until each baby can function on his or her own.

Monozygotic twins are genetically identical and are almost always both the same gender. There are documented cases where an original XXY zygote may form monozygotic male and female twins by dropping the Y chromosome for one twin and the extra X chromosome for the other. Monozygotic twins generally look strikingly similar. Fine physical details such as fingerprints and hair patterns most likely differ. The likelihood of a single fertilisation



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