Living with Our Genes is a book by Dr. Dean Hamer, a researcher at the National Institutes of Health who gained wide recognition for his studies of behavioral genetics. The book was written in 1998 before the human genome was fully sequenced. It’s still a great read. This post has some notes from the book.
• A study of twins raised apart showed that identical twins had a correlation of 0.54 for novelty seeking, while fraternal twins scored 0.32. This gave a heritability estimate of about 59 percent, indistinguishable from the estimate for twins raised together.
• A study found that people with long versions of the D4DR gene scored on average 0.4 standard deviations higher for estimated novelty-seeking scores than did people with the shorter versions.
• A study of college students who had been dating for three months or more found that there was a strong relationship between similar novelty-seeking scores and contentment with the relationship and sexual satisfaction.
• A fast fetal heart rate is a sign that an infant will probably be fidgety and whiny, which often means a high level of harm avoidance in adulthood. By contrast, a low fetal heart rate is a sign of an infant more likely to be smiley and cooey, a way of being that often matures into a rosier view of life.
• Research on hundreds of children has shown that harm avoidance is one of the earliest expressed and most persistent aspects of personality.
• There is a consistent pattern of more right frontal activation in inhibited or shy children and more left frontal activation in uninhibited or bold children. The correlation continues into adulthood: highly anxious adults show more right frontal activity than less anxious individuals. The right frontal adults are also more likely to show signs of depression.
• Studies show that the saliva of shyer girls has two times more of the stress hormone cortisol than that of bold girls.
• A study of twins by Adam Matheng at the University of Louisville found that identical two-year-olds were more similar than fraternal twins in their reaction to a stranger.
• A study at the University of Virginia of 350 paris of seven-year-old twins estimated that 50 percent of shy, fearful, and inhibited behavior was inherited.
• Scientists at the Institute for Behavior Genetics found that 50 to 60 percent of shyness was inherited in twin babies. When they focused on the most bold and most shy children, the heritability rate rose to 70 to 90 percent – one of the highest rates for any aspect of behavior and probably the reason it doesn’t change much during a lifetime.
• Studies at the Medical College of Virginia found that the coinheritance rate for anxiety and depression was more than 99 percent the same.
• Systematic studies have shown that the parents and siblings of bipolar people have a 10-fold increase in disease rate. The identical twin of a person with bipolar disorder has nearly a 40-fold increased chance of having the disease.
• In twin studies of juvenile delinquency conducted in North America, England, and Japan, the correlations were 91 percent for identical twins and 73 percent for fraternal twins.
• A study in Denmark calculated heritability of 76 percent for repeated property crimes and 50 percent for violent offenses against persons.
• A study of discharged marines found that those who exhibited signs of excessive violence and psychopathic deviance had decreased levels of 5-hydroxyindoleacetic acid, a breakdown product of the brain transmitter serotonin. Low levels of the same chemical were also found in a broad range of violent or aggressive people, including prisoners convicted for impulsive and aggressive crimes, children who tortured animals, disruptive school children expression hostility towards their mothers and aggression toward others, and men with unusually high scores for aggression, irritability, hostility, or psychopathic deviance.
• A study of college fraternity brothers found that the leaders had higher serotonin levels than new pledges.
• A study of over 4,000 U.S. military veterans found that the ones who ranked in the top 10 percent for testosterone had significantly increased antisocial behavior: assault, physical aggression, going AWOL, trouble with parents, teachers, and peers. They also had increased drug and alcohol use and multiple sexual partners.
• A study of mice who lacked the gene to make nitric oxide found that they were much more violent.
• A study of mice with a knockout for the monoamine oxidase A gene bit and attacked other mice without provocation.
• When drug-addicted animals were tested with PET scans, their brains showed a hot spot of metabolic activity directly over the nucleus accumbens.
• PET scans of recovering cocaine addicts show that the dopamine circuit is pretty much back to normal after one year of abstinence.
• Studies in the United States and Sweden of children who were put up for adoption found that the biological children of alcoholics had a four times higher risk of becoming alcoholics themselves than the children of nonalcoholics, even though they’d been separated from their alcoholic parents after the first few weeks of life.
• A study of boys who were given small doses of alcohol found that the sons of nonalcoholics swayed four times as much as the sons of alcoholics. The sons of alcoholics also were less likely to feel nausea, dizziness, or drunk.
• Heritability estimates for smoking range from 28 to 84 percent, and a summary of the results shows a mean heritability of 53 percent.
• A study at the University of Western Ontario found that a novelty-seeking score was a better predictor of the number of sexual partners than was any other factor, including physical attractiveness, masculinity, age, or general interest in sex.
• A study of straight men found that the ones with the long form of the D4DR gene, the high novelty seekers, were six times more likely to have slept with another man than those with a short gene.
• An analysis of 158 paris of twins raised apart indicates that IQ has a heritability of 75 percent.
• The world’s literature on twin, family, and adoption studies gives an estimate of about 50 percent for the heritability of IQ.
• In a French study, children adopted into homes with high socioeconomic levels increased IQ scores by an average of 12 points.
• A study at the University of Alabama designed an intensive program for the children of intellectually disabled mothers. The program reduced intellectual disability in this genetically vulnerable population by as much as 50 percent.
• Data from twin studies involving subjects between the ages of 11 and 88 showed that the heritability of IQ increases over the lifetime from about 50 percent to as high as 80 percent.
• A study of 93 pairs of identical twins who were raised in different households found the body mass index was 70 percent correlated.
• Obese mice who were injected with leptin lost 40 percent of their weight within one month.
• Inheriting two copies of the ApoE4 allele is associated with an eightfold increase in the chance of Alzheimer’s disease and a 90 percent probability of Alzheimer’s by age 90.
