Since it emerged as a science in the early 20th century, psychology has focused on the environmental causes of behavior. And the family, or nurture, is thought to be the primary source that determines who we are. But, in the 1960s, this view started getting challenged by geneticists who began to realize that shared traits between parents and children could be due to nature rather than nurture alone. And since then, long-term twin and adoption studies have shown the immense genetic contribution to our psychological differences. The word genetic in this book, as Plomin defines it in the beginning, refers to differences in DNA sequence, the 3 billion steps or basis of DNA we inherit from our parents at the conception moment. The genetic influence goes beyond psychology, but Plomin focuses on psychology first because he is a psychologist, and second the implications of the DNA revolution for psychology are more personal. Psychology is the essence of who we are. Plomin states: "Genetics is the most important factor shaping who we are. It explains more of the psychological differences between us than everything else put together." Findings from the genetic studies of twins and adoptees are transformative for psychology and society. Plomin thinks these findings made it possible to disentangle nature and nurture for the first time. One of the discoveries is that the correlations between environmental measures and psychological traits cannot be assumed to be purely environmental. Instead, Genetics is responsible for half of these correlations. The effect of parenting on children's psychological development, for example, was considered an environmental measure, but it turned out that genes' role in that is significant; We choose, adjust, and even create our experiences partly based on our genetic propensities. Environmental effects are critical, but as we will see in the book, they are predominantly random, unsystematic, and unstable, so we cannot do much about them. The book indicates that the inherited 3 billion steps of DNA differences from our parents are the consistent lifelong source of psychological individuality. We are the same as every other human for more than 99 percent of these 3 billion steps or basis, and less than 1 percent of the DNA steps that vary among us is what makes us who we are as individuals.
What is the relative importance of nature and nurture for psychological traits? Although the book is about psychological traits, Plomin thinks it is helpful to contrast psychological traits with a few physical traits initially. And he asks you as a reader to note your opinion about nature and nurture by rating a table of fourteen traits, such as eye color, weight, breast cancer, schizophrenia, autism, school achievement, and intelligence. How heritable do you think they are? If you believe the trait shows no genetic influence, rate it 0 percent. And if you feel that the trait is entirely due to genetics, rate it 100 percent. Then you compare your rating to those from a 2017 survey of 5000 young adults in the UK. In short, the table shows that decades of genetic research indicate that inherited DNA differences account for about 50 percent of all psychological differences, including all types of psychopathology, like mood disorders, anxiety, attention deficit, obsessive-compulsive personality, and drug addiction. These estimates of genetic influence are called heritability, which describes how much inherited DNA differences explain individual differences. As a specie, we all walk on two legs, and we all have eyes in the front of our heads to perceive depth. Such characteristics are programmed by the 99 percent of our DNA that do not differ between us and that we call innate or inborn, universal features shaped by evolution, and they don't vary. In contrast, heritability is about the 1 percent of the DNA that differs between us and contributes to our differences. Heritability indexes the scope of a trait is heritable. For example, to which extent weight is heritable. The weight heritability is 70 percent, which means their inherited DNA sequence can explain 70 percent of differences between people in weight. The other 30 percent could be due to environmental influences like diet and exercise. But, as the book argues, environmental factors are unsystematic and random experiences over which we have little control. Plomin is trying to be specific about heritability. Heritability is not something fixed and constant like gravity or the speed of light. Instead, it is statistics describing a particular population at an exact time with its mix of genetic and environmental influences. Heritability of body weight, for instance, is more significant in wealthier countries such as the US than in poorer countries because richer countries have greater access to fast food and high-energy snacks, stimulating people with a greater genetic propensity to gain weight. Back to the survey table where the average rating shows that most people accept the role of genetic influences, but there are huge discrepancies between what most people think and what research tells us. Plomin reviews a few of these discrepancies, the biggest for breast cancer; 53 percent of people in the survey believe that breast cancer is heritable, but research shows that it is the least heritable of the fourteen traits. In breast cancer, heritability is only 10 percent of the answer; even though identical twins are like clones, the breast cancer rate for a woman with an identical twin with breast cancer is only 15 percent. Because identical twins inherit the same DNA, breast cancer must be due to environmental causes we do not know about or non-inherited mutations pop up by chance in a particular cell. Although DNA mutations often cause breast cancer and other kinds of cancer, we don't inherit these DNA mistakes from our parents nor pass them on to our children. Another noteworthy discrepancy in the survey is what people think about body weight heritability and what research tells us. On average, people rated weight as 40 percent heritable, but the estimation made by geneticists is 70 percent. In addition, people assume that weight is a subject of willpower, whereas cultures blame overweight people as though they lack the self-control to stop eating. But, of course, finding that 70 percent of differences in body weight are triggered by inherited DNA does not contradict the fact that anyone can lose weight if they stop eating or don't have access to food. We know it is much easier for some people to gain weight and much harder to lose it for genetic reasons. How did people rate the psychological traits in the survey? The average rate was 28 percent compared to the research estimate of 58 percent. One of the most considerable discrepancies between people's ratings and the research results was the rating of school achievement. People's average rating of the heritability of school achievement was 29 percent, while the research estimated that 60 percent of schools' achievement performance is due to inherited DNA differences. Plomin says that the survey was critical for him in writing the book because, in the past, both mainstream psychological research and public opinion ignored the importance of genetic influence. And even though they underestimate its impact, most people today accept the role of DNA in psychological traits.
How do we know that DNA makes us who we are?
Studying adoption is a way to disentangle nature from nurture; we can see how similar adopted children are to their biological parents, who adopted them away directly after birth. The adoption design is robust in disentangling the influence of nurture and nature. It can include the genetic parents who gave birth to the child, environmental parents who adopted the child, and the environmental-plus-genetic parents, which refers to the usual situation where parents share nature and nurture with their children. As a consequence of the sexual revolution, adoption in the US peaked in the early 1970s, when the percentage of babies born to unmarried women jumped from less than 4 percent before 1960 to over 15 percent by the 1970s. In 1974, Plomin decided to start a long-term longitudinal adoption study of psychological development. The adopted away children then didn't see their biological mothers after the first week of life. In the 1970s, pregnant young unmarried women often stayed in special-care homes managed by adoption agencies; most teenagers left their lives without anyone knowing. They only wanted to give birth, adopt the child, and return to everyday life. For two years, Plomin worked on conducting tests of several dozens of unwed mothers in care homes during pregnancy. The measures he collected included cognitive tests and questionaries about personality and psychopathology. He also gathered information about education, smoking and alcohol consumption, height, and weight. The study was called Colorado Adoption Project CAP and aimed to study 250 adoptive parents and their adoptive children and 250 matched 'control' parents, parents who gave birth and raised their children. The CAP children were examined at ages seven, twelve, and sixteen. At age sixteen, more than 90 percent of the CAP children completed the same assessments their parents had completed sixteen years earlier. The study continues today with the children now in their forties.
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