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Even the impressive data from personality assessments are clouded by the reunion of
many of the separated twins some years before they were tested. Moreover, when adoption
agencies are involved, separated twins tend to be placed in similar homes. Despite these
criticisms, the striking twin-study results helped shift scientific thinking toward a greater
appreciation of genetic influences.

Biological Versus Adoptive Relatives

For behavior geneticists, nature’s second real-life study — adoption — creates two groups:
genetic relatives (biological parents and siblings) and environmental relatives (adoptive par-
ents and siblings). For personality or any other given trait, we can therefore ask whether
adopted children are more like their biological parents, who contributed their genes, or their
Distributed by Bedford, Freeman & Worth Publishers. Not for redistribution.
adoptive parents, who contributed their home environment. And while sharing that home
environment, do adopted siblings come to share traits?
The stunning finding from studies of hundreds of adoptive families is that, apart from
Copyright © Bedford, Freeman & Worth Publishers.
identical twins, people who grow up together — whether biologically related or not — do
not much resemble one another in personality (McGue & Bouchard, 1998; Plomin, 2011;
Rowe, 1990). On personality traits such as extraversion and agreeableness, for example,
people who have been adopted are more similar to their biological parents than to their
caregiving adoptive parents.
The finding is important enough to bear repeating: The normal range of environments
shared by a family’s children has little discernible impact on their personalities. Two adopted chil-
dren raised in the same home are no more likely to share personality traits with each other
than with the child down the block.
Heredity shapes other primates’ personalities, too. For example, macaque monkeys
raised by foster mothers exhibit social behaviors that resemble those of their biological,
rather than foster, mothers (Maestripieri, 2003).
Why are children in the same family so different? Why does a shared family environment
have so little effect on children’s personalities? Is it because each sibling experiences unique
peer influences and life events? Because sibling relationships ricochet off each other, ampli-
fying their differences? Because siblings — despite sharing half their genes — have very
different combinations of genes and may evoke very different kinds of parenting? Such
questions fuel behavior geneticists’ curiosity.
The genetic leash may limit the family environment’s influence on personality, but it
does not mean that adoptive parenting is a fruitless venture. One study followed more than
3000 Swedish children with at least one biological parent who had depression. Compared
to their not-adopted siblings, those raised by an adoptive family were about 20 percent less
likely to develop depression (Kendler et al., 2020a). As an adoptive parent, I [ND] especially
find it heartening to know that parents do influence their children’s attitudes, values, man-
ners, politics, education, and faith (Gould et al., 2019; Kandler & Riemann, 2013). This was
dramatically illustrated during World War II by separated identical twins Jack Yufe, a Jew,
and Oskar Stöhr, a member of Germany’s Hitler Youth. After later reuniting, Oskar mused
to Jack: “If we had been switched, I would have been the Jew, and you would have been the
Tim Clayton/Corbis/Getty Images their children — matters!
Nazi” (Segal, 2005, p. 70). Parenting — and the cultural environments in which parents raise

Moreover, child neglect and abuse and even parental divorce are rare in adoptive
homes. (Adoptive parents are carefully screened; biological parents are not.) One study
looked at the parenting of siblings being raised apart — some with their biological mother,
some with an adoptive mother (Natsuaki et al., 2019). Compared with the biological moth-
Adoption matters Olympic gold ers, the adoptive mothers used gentler parenting, gave more guidance, and experienced less
medal gymnast Simone Biles benefited depression. It is not surprising, then, that studies have shown that, despite a slightly greater
from one of the biggest gifts of love:
adoption. risk of psychological disorder, most adopted children thrive, especially when adopted as

14 Unit 1 Biological Bases of Behavior

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