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Genetic Influences on Personality?
To what extent are personality traits also genetic traits? If we look at non-human animals, such as dogs, we can see the results of life experiences (for example, a dog given lots of love as a puppy will have a sweet personality) but we can also see breed personalities.
Dogs were selectively bred by humans to produce different breeds with specific characteristics. One result was the emergence of distinct, identifiable personalities in different dog breeds.
Afghans are aloof, beagles are irrepressibly friendly, certain sheep dogs are bred to be shy of strangers, golden retrievers love everybody, and so forth. Each breed has its behavioral traits.
What are examples of breed personalities in dogs? What gave the Freedmans a unique background for their work?
In Chapter 8 (Animal Behavior and Cognition) we saw many examples of genetically determined traits, such as fixed action patterns (species-typical behaviors). We also reviewed reasons to be cautious about simplistic evolutionary reasoning.
For example: one genetic change can have multiple behavioral and other effects (pleiotropy), and most traits (physical or behavioral) are the results of multiple gene systems, not just one. Having said that: genetic influences on personality are clear and undeniable, in both humans and non-human animals.
One implication is that clones should have very similar personalities, and they do. For example, a manager who worked for Infigen, a biotech company that cloned 193 cattle said, "These clones have the same personalities... They bellow all the time. They're easygoing, friendly" (Kristof, 2002).
A husband-wife research team, Daniel and Nina Freedman, grew curious about "breed differences" in humans (Freedman and Freedman, 1985). Daniel raised and trained dogs for a living. Nina was a developmental psychologist.
Daniel's ancestors came from Europe, Nina's parents came from China. They noticed big differences in the way their families behaved.
For example, Daniel's family was animated around the dinner table, Nina's was quiet. The obvious explanation is that Chinese and American cultures have different rules of conduct at mealtime.
But where do the rules come from? The Freedmans started wondering if the difference might be based partly on genetics.
The Freedmans decided to make behavioral comparisons between infants of differing ethnic backgrounds. They figured that newborn infants were still too young to be influenced by culture, so behavioral differences in newborns would probably be due to genetic differences.
They located pregnant women of European or Chinese ancestry in the San Francisco area. The two groups were matched on such variables as years spent in the United States, quality of prenatal care, and income level. The babies of these mothers were tested soon after birth.
Sure enough, there were big differences in temperament. The American babies cried loudly when angry and were hard to calm. The Chinese babies stopped crying almost immediately when picked up.
The American babies turned their heads vigorously if a cloth was placed on the nose. (The Freedmans pointed out this was listed in American pediatric textbooks as the "normal" response.) However, the Chinese babies merely breathed through their mouths when their noses were covered.
Why was it significant to the Freedmans that Navajo babies resembled Chinese babies in behavior?
In further research the Freedmans found striking similarities in the behavior of Chinese and Navajo babies. The Navajo babies, when tested, resembled the Chinese babies in every way, but were unlike the Caucasian babies.
This is significant because Navajos are descended from people of Asia who migrated over the land bridge between Alaska and Siberia around 10,000 years ago. One might expect genetically based behavior patterns to be similar in the two groups.
The Freedmans' research by itself would not be enough to prove that temperament is "in the genes." However, there are many sources of evidence that point to the same conclusion.
In the 1980s, several personality theorists, such as Buss and Plomin, addressed the genetic basis of personality traits and temperament. By 1987 Plomin was calling the finding of genetic influences on personality "the single most important finding in behavioral genetics in the last decade" (Holden, 1987).
Bouchard's Research on Identical Twins
Thomas Bouchard of the University of Minnesota did the most famous research on genetic influences in humans. He studied identical twins separated since birth.
Identical twins come from a single egg, fertilized by a single sperm, which splits after the egg starts to develop. Therefore identical twins are more genetically identical than any other humans.
By studying identical twins who were separated at birth and raised by different families, Bouchard could see what similarities emerge from the same genotype despite different family environments. Such similarities might be heavily influenced by genes.
What was Bouchard's research? How did it start?
Bouchard's data set was unique, probably a one-time event in history. Adoption agencies no longer break up sets of identical twins.
Bouchard's project started when he read news reports of two identical twins reunited after a lifetime apart: James Lewis and James Springer were separated weeks after birth. When they were brought together for interviews and measurements, an extraordinary collection of coincidences emerged.
Both of the "Jim twins" had married and divorced women named Linda. Both had second marriages with women named Betty.
Both had police training and worked part-time with law enforcement agencies. Both had childhood pets named Toy.
They had identical drinking and smoking patterns, and both chewed their fingernails to the nub. Their first-born sons were named James Alan Lewis and James Allan Springer. (Holden, 1980)
How were Oskar and Jack raised? In what ways were they similar?
Bouchard was inspired to look for more cases of identical twins reared apart, and he found them. Holden reports:
In one case, identical twin babies (Oskar and Jack) were raised in extremely different cultures. The two were born in Trinidad and separated shortly after birth. After that, their childhoods were very different.
The mother took Oskar back to Germany, where his grandmother raised him as a Catholic and a Nazi youth. Jack was raised in the Caribbean as a Jew, by his father, and spent part of his youth on an Israeli kibbutz.
But similarities started cropping up as soon as Oskar arrived at the airport. Both were wearing wire-
They share idiosyncrasies galore: they like spicy foods and sweet liqueurs, are absentminded, have a habit of falling asleep in front of the television, think it's funny to sneeze in a crowd of strangers...
[They] flush the toilet before using it, store rubber bands on their wrists, read magazines from back to front, dip buttered toast in their coffee... Bouchard professed himself struck by the similarities in their mannerisms, the questions they asked, their "temperament, tempo, the way they do things." (Holden, 1980)
Farber (1981) reviewed 121 case studies and confirmed the existence of "remarkable–sometimes unnerving–similarities" in many dimensions. He wrote:
What twins seem to share most is what might be called personality—those peculiarities of manner and personal preference... laughter, vocational interests, posture, tastes in clothes, choices of names.
In what respects were twins most identical?
Australian twin researcher David Hay ran one of the world's largest twin studies, collecting data from 587 sets of twins for over a decade. Hay noted that identical twins are often "mirror images" of each other, with facial structure, fingerprints, and dominant hand reversed.
This mirror-image duplication extends to the brain. If one twin is right-hemisphere dominant, the other tends to be left-hemisphere dominant.
Because the two sides of the brain are somewhat specialized for different things, mirror-image twins may think in different ways. One may excel in math, the other in language arts, for example.
However, even twins who think differently can be very similar in temperament and other personality variables. Bouchard found that identical twins reared apart were not outstandingly similar in IQ scores or standardized personality tests, but (as Farber pointed out) he did find striking similarities in mannerisms.
What did Hay find out about the brains of twins?
Examples of quirks or mannerisms are: wearing rubber bands on the wrists, or reading magazines backwards (like Oskar and Jack, above), personal choices (such as names for pets or children, or clothing styles), and expressive social behavior (shyness or social ease, laughter, facial expressions and posture). If this is personality, then Bouchard's findings support strong genetic influences on personality.
Such influences are now almost universally accepted. Turkheimer (2000) summarized "three laws of behavior genetics":
- First Law. All human behavioral traits are heritable.
- Second Law. The effect of being raised in the same family is smaller than the effect of genes.
- Third Law. A substantial portion of the variation in complex human behavioral traits is not accounted for by the effects of genes or families.
Criticism of behavior genetics changed over the years, Turkheimer pointed out. Initially, many social scientists were skeptical about genetic influences. Many expected that further research would show most personality traits were caused by the environment.
That criticism disappeared. Scientists came to recognize there was "no versus" when evaluating genetic and environmental contributions to development: both were always present, and all traits were influenced by inheritance.
The new criticism was that behavior genetics could not explain how genes caused developmental differences, because genetics and environmental factors were too tangled up.
Lab experiments with non-human animals could pinpoint genetic influences, because genes could be altered one by one and experimental controls could rule out alternative explanations. Experimenters could change a single gene in cloned animals and observe the effects. Needless to say, in humans such research was not permissable.
Therefore (Turkheimer's point) the argument had shifted. No longer was anybody denying genetic effects; instead, they were complaining that research could not pinpoint exact influences in humans.
How did criticisms of genetic explanations shift?
Why, critics asked, are children from the same family so often very different? One obvious explanation could be that siblings react to each other's presence.
To some extent, each child seeks a unique niche. If older brother excelled in sports, younger sister might strive to excel in academics. That is why it was important for Bouchard to study identical twins reared apart.
Plomin and Daniels (1987) were committed behavioral geneticists, but they agreed that specific genetic explanations in humans were elusive. They wrote an article, "Why are children in the same family so different from one another?" The answer, they suggested, was that differences might be due to unusual or one-time influences.
One gloomy prospect is that the salient environment might be unsystematic, idiosyncratic, or serendipitous events such as accidents, illnesses, or other traumas ....
Turkheimer said that was not a gloomy prospect, except for researchers wanting clear results. Idiosyncratic influences included good things, such as unique opportunities, extraordinary teachers, or new technology.
Therefore, Turkheimer maintained, this is not a problem, and we do not wish to eliminate such "idiosyncratic" effects on human development. They are some of the most interesting influences to study.
Farber, S. (1981, January). Telltale behavior of twins. Psychology Today, pp.58-62,79
Freedman, D. G. & Freedman, N. (1979) Ethnic differences in babies. Human Nature, 2, 36-44.
Holden, C. (1980). Identical twins reared apart. Science, 210, 1323-1327.
Holden, C. (1987, September 27) Does biology make personality? Washington Post. Retrieved from: https://www.washingtonpost.com/
Kristof, N. D. (2002, July 23) Interview with a humanoid. New York Times. Retrieved from: https://www.nytimes.
Plomin, R. & Daniels, D. (1987). Why are children in the same family so different from one another? Behavioral and Brain Sciences, 10, 1-60.
Turkheimer, E. Three laws of behavior genetics and what they mean. Current Directions in Psychology, 9, 160-164.
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