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Fraternal (dizygotic) twins develop from two separate fertilized eggs. Although they
were wombmates, they are genetically no more similar than ordinary siblings.
Shared genes can translate into shared experiences. A person whose identical twin has
autism spectrum disorder, for example, has about a 3 in 4 risk of being similarly diagnosed.
If the affected twin is fraternal, the co-twin has about a 1 in 3 risk (Tick et al., 2016). To
study the effects of genes and environments, several thousand medical and psychological
researchers have studied nearly 15 million identical and fraternal twin pairs (Polderman
et al., 2015).
Are genetically identical twins also behaviorally more similar than fraternal twins?
Compared with fraternal twins, identical twins are much more alike in their personality,
their politics, and even the age when they begin using marijuana, if they do so (Hufer et al.,
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2020; Minic˘a et al., 2018).
Identical twins, more than fraternal twins, look alike — so much so that most have dif-
ficulty distinguishing a flashed photo of their own face from their co-twin’s face (Martini
et al., 2015). Does that mean people’s responses to their looks account for their similarities?
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No. In a clever approach, researcher (and fraternal twin) Nancy Segal (2013) compared per-
sonality similarity between identical twins and unrelated look-alike pairs. Only the identical
twins reported similar personalities.
Other studies have shown that identical twins whose parents treated them alike (for
example, dressing them identically) are not psychologically more alike than other identical
twins (Kendler et al., 1994; Loehlin & Nichols, 1976). In explaining individual differences,
identical genes matter more than identical jeans.

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Do look-alikes act alike? Genetically unrelated look-alikes, called doppelgangers,
tend not to have notably similar personalities (Segal, 2013). Amazingly, these two bearded,
red-haired, 6 foot 4 inch tall, minor league baseball pitchers also share the same unusual
name — Brady Feigl — but are unrelated and grew up unaware of each other.

Separated Twins

fraternal (dizygotic) twins Imagine the following science fiction experiment: A mad scientist, given two pairs of iden-
individuals who developed from tical twins, swaps one in each pair. The resulting pairs are then raised in separate environ-
separate fertilized eggs. They ments as if they were fraternal twins. Better yet, consider a true story (Dominus, 2015; Segal
are genetically no closer than
ordinary siblings, but they shared & Montoya, 2018).
a prenatal environment. In 2015, William Velasco was working as a butcher in Bogotá, Colombia. One day, cus-
tomer Laura Vega Garzón mistook him for her colleague, Jorge, who looked the same — same

12 Unit 1 Biological Bases of Behavior

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