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Making Sense of People
Decoding The Mysteries of Personality
Francis Galton, Charles Darwin's cousin, was first to use twin studies to try to distinguish between the effects of nature (genetic inheritance) and nurture (influence of your upbringing). He (correctly) assumed that identical twins were genetically identical whereas non-identical twins were no more similar than siblings born at different times. In 1875 he reported that 35 sets of identical twins showed much greater behavioural similarities than 20 sets of non-identical twins. He also noticed that adopted children placed with gifted parents turned out no more gifted than ordinary families even though they were provided with a privileged environment.
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When these and other twin studies were published, critics suggested that perhaps ID twins more similar because they were treated exactly the same. This was disproved by studies of ID twins raised apart. Thomas Bouchard of University of Minnesota tracked down 100 pairs of such twins, some of whom had been raised in different countries and/or cultures. They found that the separation made virtually no difference. Even though their speech often reflected different upbringing, their personalities, temperaments, hobbies, interests and occupations were about as similar as ID twins raised together.
And, studies of genetically unrelated children adopted and raised in same family also showed no effect of shared environment.
So our genes control many of our personality traits. But it is the action of multiple gene variations.
John DeFries studied one mouse trait: the inclination to explore strange and potentially dangerous territory. This personality trait is a continuum from high anxiety (freeze if put into any new open space) to adventurous (sniff around and explore). DeFries measured all mice in 10 litters and gave them a score ranging from most A for Anxious to F for Fearless. Then he bred the most active (F) male and female from each litter, and did same for most anxious (A) mice. Repeated with each generation. Took him 10 years to get through 30 generations. After 30 generations the F litters would roam freely while the A ones would huddle in the corner of the box. The pattern of change was a steadily rising (or falling) graph. This gradual change implied that variants of many genes act together, and the effects keep adding up as more variants are selected in each generation.
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As Darwin recognized, the environment keeps selecting for variants that increase fitness. All humans started out black in Africa, but as they moved away from the tropics, the need to absorb Vit D exerted a relentless selective force on gene variants that eventually made Northern Europeans white.
Extroverts enjoy the pleasures of intense engagement with others, and the opportunities provided to those who take charge, and they get more sex, which usually means more offspring. Darwin suggests those genes would predominate. But there are risks - high engagement means jealousy and insurrection, excitement seeking increases chances of death or being locked up. Every personality type still in human existence has benefits and costs. In some situations one will thrive, but will wither in other environments.
Theories of child development used to be based on the idea that we went through a series of stages from total infant dependency to the independent adult life. Before understood how brain changes during transition to adulthood, assumed that personality differences arose because of different experiences.
Teenagers are in process of literally restructuring and rewiring their brains. The structure of the prefrontal cortex and its connections to the amygdala change. And during this process teenagers are very open to peer influence.
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