POLICY & ETHICS
In the Nature-
Environmental influences are important,
too, but they are largely unsystematic,
unstable and idiosyncratic
hen psychology emerged as a science in
the early 20th century, it focused on nur-
ture, the environmental causes of behav-
ior. Environmentalism—not the ecological kind, but
rather the view that we are what we learn—domi-
nated psychology for decades. From Freud on-
ward, the family environment was assumed to be
the key factor in determining who we are. In the
1960s geneticists began to challenge this view.
Psychological traits such as mental illness clearly
run in families, but there was a gradual recogni-
tion that family resemblance could be due to na-
ture (genetics) rather than nurture (environment)
alone, because children are 50 percent similar ge-
netically to their parents.
During the past four decades, scientists have con-
ducted long-term studies on special relatives like twins
and adoptees to test the effects of nature and nurture.
This research has built a mountain of evidence show-
ing that genetics contributes importantly to all psycho-
logical differences between us. In fact, inherited DNA
differences account for about 50 percent of the differ-
ences between us, in our personality, mental health
and illness, and cognitive abilities and disabilities.
The word “genetic” can mean several things, but
here it refers to differences in DNA sequence, the
three billion steps in the spiral staircase of DNA that
we inherit from our parents at the moment of concep-
tion. It is mind-boggling to think about the long reach
Robert Plomin is professor of behavioral genetics at the Institute of Psychiatry,
Psychology & Neuroscience at King’s College London. He previously held
positions at the University of Colorado Boulder and Pennsylvania State University.
He was elected a fellow of the Academy of Medical Sciences and of the British
Academy for his twin studies and his groundbreaking work in behavioral genetics.
His new book from The MIT Press is Blueprint: How DNA Makes Us Who We Are.