A biological experiment: twins
In 1994 Plomin started another long-term longitudinal study in the UK, this time a study of twins. The study is called the Twins Early Development Study (TEDS). The twin method is based on comparing identical and fraternal twins. Identical twins have identical DNA sequences, but fraternal twins share only 50 percent of the DNA differences. The twin method is based on comparing identical and fraternal twins. There are two types of twins, identical twins, who come from the same fertilized egg, and fraternal twins, which come from two eggs fertilized simultaneously. And when identical twins share 100 percent of the DNA, fraternal twins, like any brother or sister, are 50 percent similar genetically.
TEDS obtained DNA from more than 12,000 twins, and their families were part of the study when the twins were aged two, three, four, seven, eight, nine, ten, twelve, fourteen, and sixteen—then studying the twins again as they emerge into adulthood at the age of twenty-one. Although TEDS focused on cognitive and language development, it collected questionnaire data about home, health, and school environments. And over twenty years, 55 million data items have been gathered from parents, teachers, and twins.
In the cognitive field, for example, TEDS found that children's school achievement in subjects from the humanities to science is heritable to a high degree. The study also found that differences in language skills, such as fluency and learning a second language, are substantially heritable. In addition, the study found that a lack of empathy in childhood and disregard for others are highly heritable. Furthermore, similar results showed substantial heritability even with aspects of well-being such as life satisfaction and happiness. To estimate heritability, we need statistics, and there are two primary statistics of individuality: variance and covariance. Plomin says both are crucial for understanding genetics and interpreting all scientific data on individuality. Variance is a statistic that explains how much people differ, whereas covariance describes similarity. We are familiar with the term 'correlation,' which describes the relationship between traits such as weight and height. In this case, the correlation indicates the proportion of the variance that covaries. For example, if the correlation between weight and height is 0.0, that means no similarity, whereas a correlation of 1.0 means complete resemblance. Now, if it is evident that taller people weigh more, then the correlation is not zero, but how strong is it? Here, a correlation of 0.1 is small, 0.3 is moderate, and 0.5 is significant. And if you like to know, the correlation between weight and height is 0.6. But, in genetics, instead of correlating traits like weight and height, they correlate a trait of one member of the twin pair with the same trait in the other twin, so if the correlation is 0.0, that means the twins have no resemblance at all. While a correlation of 1.0 means they are similar.
We can use the difference between identical and fraternal twins to estimate heritability, and the most direct estimate comes from the correlation for identical twins reared apart. Although identical twins raised separately are rare, several hundred have been studied, and across all studies, the identical twin's correlation for weight was 0.75. That means that 75 percent of the differences between people in weight (variance) are shared (covariance) by these identical twins who did not grow up in the same family. Is the similarity in weight between parents and children an indication of nature or nurture? The straightforward answer from genetics says that even though these twins are adopted away from their mothers at birth, they are similar in weight to their birth mothers as it is for children whose biological mothers rear. Even though adoptive parents put food on the table, their adopted children are not similar to them in weight. The heritability of weight is 70 percent, which implies that 30 percent of the differences in weight are environmental forces. But these environmental influences are not just sharing the family environment. As the book explained, and we will come on later, the family environment has little effect on individual differences, not just in weight, but across all traits. A Hug number of studies relied on adoption and twin methods to explore the impact of DNA on thousands of traits throughout biological and medical sciences, such as the structural and functional measures of the brain and heart. Across all traits, the average heritability is 50 percent
The nature of nurture
When we think about nurture, we imagine parents cooing and cuddling their babies. However, Freud believed parenting is the main component in a child's development. Therefore, he focused on specific aspects of parenting, such as how breastfeeding and toilet training affect sexual identity. And since Freud, thousands of behavioral science studies have examined other parenting aspects, like discipline and warmth, as environmental elements influence the child's development. Developmental psychologists study parenting to ask whether differences in parenting cause differences in children's outcomes. For example, are differences in parental warmth affect how well their children are later in life? How much do parents read to their kids correlates with how well they will read at school? Is hanging out with deviant peers associated with bad outcomes, such as using drugs? It is reasonable to assume that the relationships between the environment and the psychological measures are caused environmentally, but as plausible as these causal interpretations appear. We should always remember that the path from correlation to causation is full of traps. Genetics could be the third factor contributing to the correlation between parents who read to their children and their children's reading ability at school. And this is what Plomin means by nature of nurture. Since parents and their children are 50 percent bonded genetically, parents who enjoy reading may have children who like to read. The early example of nature of nurture was what psychologists call stressful life events, such as breaking up with a partner, financial difficulties, illness, and problems at work. People vary in how they respond to such events and experience the same event very differently. The first genetic analysis of stressful life events was in 1990 when Plomin studied middle-aged twins from Sweden; twins reared apart and as well as twins reared together, in a study called the Swedish Adoption/Twin Study of Aging (SATSA). The surprise was that identical twins were twice as similar as fraternal twins in their scores on the measure of life events, such as a change in relationships, financial status, and sickness. People differ in what they are willing to call a severe financial difficulty or illness or difficulties of relationship breakdown. And the personality is involved in how much these events affected them; for example, optimists might see these events through rosy glasses, while pessimists see them in shades of grey. What about stressful events independent of our perception? Divorce is an example of an objective most stressful life event for most people. A study of 1500 pairs of adult twins suggests substantial genetic influences on divorce. Research has shown that certain personality traits account for a third of the genetic impact on divorce. For example, a recent adoption study in Sweden, among a sample of 20 thousand individuals, found that the likelihood of divorce was more significant if the biological mother, who did not rear the person, had divorced later in life than if the adoptive parents raised them had become divorced. The heritability of divorce across studies is about 40 percent. However, that does not mean there is a 'divorce gene' that makes some people more likely to divorce or have poor opportunities for stable marriages. Surprisingly, the odds of divorce increase if the person is joyful and engaged with life, emotional and impulsive. Probably, and ironically, these are the same good traits that make people desirable as marriage partners in the first place. But we are not passive agents of the impulse of divorce; genetics influence us, but we are not hardwired to them. In the end, we make or break the relationship. Someone will argue that if the genetic impact on divorce is 40 percent, and still 60 percent are environmental causes, why focus on genetics? The book argues that genetics are the major systematic factor influencing divorce, while no environmental research has identified any specific factors. Moreover, genetic studies have found significant heritability for most environmental measures of the environment, including how much time children spend watching TV. Children's television watching has been used in more than 2000 studies by the 1980s to explore its effect on children's development. And how much the child watched TV was the favorite environmental measure at that time. So it is good to know most US parents during the 1980s did not restrict how much children watched TV, which kept the possibility of genetic differences open. If parents control how much their children are allowed to watch will leave no door to genetic influences. And this encouraged Plomin to study children's television viewing in the Colorado Adoption Project.
The results suggested that the genetic factor inherited from their parents can account for a third of the differences between children watching TV. And just as there are no IQ or divorce genes, there are no genes for television watching. Plomin wanted to bring the psychologist's attention to the fact that genetic differences strongly influence what has been taken as an environmental measure, like watching TV. Experiencing life events, social support, watching TV, and spending time on Facebook do not just happen to us. With all our genetic differences in personality, we differ in our propensity to experience them. Although objective measures are helpful, we should consider the importance of subjective experience. For example, some children have more accidents than others; the number of children's scrapes and bruises shows the genetic influence. Another example is reckless drivers often cause car crashes, driving fast or under the influence of alcohol. Of course, accidents sometimes happen, but genetic differences in personality can increase the probability of accidents. Nonetheless, our psychological reactions to such events can be influenced by our genes. Psychologists in the past believed that the environment passively influences us, but, as Plomin claims, genetic research suggests the nature of nurture implies a more active model of experience. The environment is not something imposed on us passively. Instead, we actively perceive, select, modify, and even form environments correlated with our genetic propensities. Our genetic differences translated into personality make us experience life differently. For example, genetic differences in our vulnerability to depression affect how we interpret experiences positively or negatively.