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Sir Francis Galton coined the phrase nature versus nurture to describe the debate over the relative influence of genetics and environment on behavior, intelligence, and personality. Galton argued that heredity plays a dominant role. In contrast, philosophers like 17th-century John Locke believed children are born as a tabula rasa or “blank slate,” emphasizing the role of environment and experience.
Throughout the 20th century, psychologists provided evidence for both sides. Freud, with his psychoanalysis, and behaviorists like Pavlov, Watson, Skinner, and Bandura emphasized the impact of environment on behavior. Later, the rise of twin studies, family studies, and adoption studies helped researchers understand the complex interaction between genes and environment.
Nature refers to the genetic or internal factors that act on a person, whereas nurture refers to environmental and other external influences. While debate over the roles of nature and nurture continues, it is widely accepted that nature and nurture work together, in combination, to shape a person. For this reason, the updated term gene-environment interaction is becoming increasingly preferred in modern psychology.
Individuals inherit specific traits from their biological parents, which can range from visible traits, such as eye and hair color, skin tone, or freckles, to other traits, such as color blindness, handedness, blood type, and certain personality traits. Nature can be considered as the sum of the biological influences that are determined during conception and prenatal development (inheritance of genes and neural wiring). Nurture can involve a vast array of influences that occur after the point of conception, including a pregnant parent’s diet, environmental exposures and levels of stress. Nurture also includes childhood experiences, peer relationships, parent and family dynamics, education, and beyond. Whether or not various traits are present or expressed and the extent to which various traits are present or expressed depends largely on a deep interaction between biological and environmental factors.
Heritability
In a group of people, a particular psychological trait, such as intelligence, usually varies a lot. Differences in groups may be due to genes or the environment, and researchers use a statistic called heritability to see which has the largest influence.
Heritability is a mathematical estimate that indicates how much of a trait’s variation can be attributed to genes. There are three important principles of heritability:
- Heritability estimates don’t reveal anything about how much genes influence a person’s traits. These figures tell us only to what extent trait differences between people can be attributed to genes.
- Heritability depends on the similarity of the environment for a group of people. In a group of people who share similar environments, heritability of a particular trait may be high. However, that same trait may have low heritability in a group of people who operate in different environments.
- Even if a trait is highly heritable, it can still be influenced by environmental factors.
Example: Imagine that 10 people live in identical environments. Somehow, they experienced identical prenatal environments while in their mothers’ wombs, were raised in identical homes by parents who were identical in every way, and had all the same childhood and adulthood experiences. Suppose that these 10 people turn out to be different with respect to one trait, such as the rate at which they can wiggle their ears. Since both genes and environment can influence traits, these differences would have to be genetic, since they could not be due to differences in environment. In such a case, heritability of the ear-wiggling trait would be close to 100 percent. Now suppose some of these 10 people enter different ear-wiggling training camps. The camps vary in effectiveness, so the subjects in some camps increase their ear-wiggling rates, while other subjects remain the same. After the camp training, environment would account for some of the differences among the 10 people in ear-wiggling ability. A smaller proportion of the differences would be due to genes alone. Therefore, heritability would be lower.
Who Shares Genes?
No two people share the exact combination of genes unless they are identical twins. However, all family members share some genes with one another. The closer the biological relationship between individuals, the more genes they share. The chart below shows the percentage of genes any person shares with his or her close relatives:
|
Relationship |
Percentage of Shared Genes |
|
Identical twin |
100 |
|
Parent |
50 |
|
Sibling |
50 |
|
Nonidentical twin |
50 |
|
Grandparent |
25 |
Evolution and Natural Selection
Evolution is a change in the frequency of genes in a population over time. Evolutionary psychologists try to explain universal behaviors. They study how natural selection has encouraged certain behavior patterns to develop.
The Theory of Natural Selection
Charles Darwin (1809–1882) was a British naturalist best known for his contributions to evolutionary theory. Although others had noted that species evolved over time, Darwin first proposed the theory of natural selection to explain the process of evolution.
According to this theory, certain inherited characteristics give an organism a survival or reproductive advantage. Organisms pass on these characteristics more often than they pass on other inherited traits.
Example: The species of primates called mandrills have evolved to have bright blue rear ends, because brightly colored rumps help them attract mates and give them a reproductive advantage. Porcupines evolved to have quills, because quills help them to avoid predators and reproduce. This gives porcupines a survival advantage.
On the Origin of Species
In 1831, Darwin joined a naval expedition on a ship called the HMS Beagle as the unofficial naturalist onboard. Darwin collected many specimens during the ship’s five-year expedition around the world. After returning to England, he began developing his ideas about evolution. In 1859, Darwin published his great work, On the Origin of Species.
A characteristic that gives a reproductive advantage helps an organism to mate successfully and pass on its genes to the next generation. A characteristic that gives a survival advantage helps an organism to live long enough to reproduce and pass on its genes.
Reproduction of the Fittest
People often use the phrase “survival of the fittest” instead of “reproduction of the fittest,” but according to evolutionary theory, survival alone isn’t enough. Creatures need to survive long enough to reproduce. Reproductive success is measured by how many offspring a creature produces.
Inclusive Fitness
Another concept related to reproductive success is inclusive fitness, described by W.D. Hamilton in the 1960s. Inclusive fitness is the reproductive fitness of an individual organism plus any effect the organism has on increasing reproductive fitness in related organisms. Some researchers believe that the concept of inclusive fitness explains why certain organisms sacrifice themselves to save others in the species. According to this theory, people might risk their lives to save their children or close relatives, but not to save distant relatives or unrelated people. Because people share more genes with close relatives, saving them has more payoff in terms of passing on genes to the next generation.
Adaptations
An adaptation is an inherited characteristic that becomes prevalent in a population because it provides a survival or reproductive advantage. Because evolution occurs over a long period, an adaptation can remain in a population even after it has stopped being useful.
Example: Human beings have a genetic preference for fatty foods, which explains why fried chicken, French fries, and buttery popcorn are so popular. Evolutionary psychologists say that the preference for fatty foods derives from the days when people hunted and gathered and food was scarce. Eating high-fat foods was important because fat gave people the calories they needed. In other words, the preference for fat was adaptive. Today, in wealthy countries with abundant food and sedentary lifestyles, the preference for fat remains, despite the fact that it no longer has beneficial effects. In fact, consuming fatty foods can lead to health problems.
Mutations
Evolution relies on mutations, or small changes in genes. Mutations happen because of two events that can occur during the formation of egg and sperm cells:
- An error during copying of DNA
- Random rearrangement of small pieces of DNA in a chromosome pair
Sometimes, a mutation results in a new trait. If the individual with the mutation reproduces successfully, the mutation will be passed on. If the new trait proves advantageous, the mutated gene that caused the trait will increase in the population over a long period and thus propel evolution.
Evolutionary Psychology
Evolutionary psychology uses evolutionary theory to explain similarities in psychological characteristics. According to evolutionary psychologists, patterns of behavior have evolved through natural selection in the same way that physical characteristics have evolved. Because of natural selection, adaptive behaviors, or behaviors that increase reproductive success, are kept and passed on from one generation to the next.
Problems with Evolutionary Explanations
Scientists have used evolutionary theory to explain human behavior patterns, such as a female tendency toward monogamy and a male tendency toward promiscuity. However, other researchers argue that such explanations don’t apply well to humans because the theories stem from stereotypes. Humans behave in complex and variable ways, and factors such as culture strongly influence this behavior. Furthermore, it is difficult to tie variation in behavior to variation in reproductive success. Evolutionary explanations also raise controversy because people can use them to support various social and political agendas.
The principles of the evolutionary perspective inform a controversial and discriminatory set of beliefs and practices called eugenics. It aims to improve the quality of the human species by improving genetics using a variety of practices, which include but are not limited to:
- the required use of birth control
- genetic screenings
- required abortions
- marriage restrictions
- sterilization
The term eugenics was originally coined by Francis Galton, a half-cousin of Charles Darwin and an academic whose work and influence spanned a number of subjects. Francis Galton originally proposed in 1883 that society should use monetary rewards to incentivize marriage between individuals deemed the fittest. The principles of eugenics are widely considered to be immoral, as this requires categorizing people as either “fit” or “unfit.” Furthermore, the principles of eugenics assume that traits depend heavily on genetics and very little on the environment, ignoring the benefits of a rich and supportive environment. For example, they do not acknowledge the possibility that someone who lacks genetic priming for athleticism may become a skilled athlete through determination and access to suitable training.
Some researchers criticize evolutionary explanations because anyone can work backward from an observation to develop an evolutionary explanation. These psychologists point out that the fact that a trait exists does not necessarily mean that the trait is adaptive. The trait may have been helpful earlier in human history but did not remain adaptive, or it could be a side effect of another adaptive trait.
Types of Genetic Studies
Researchers do different kinds of studies to see whether, and to what extent, a characteristic might be genetically transmitted.
Family Studies
In family studies, researchers look at similarities among family members with respect to a particular trait. If the trait is genetically inherited, it should be similar in blood relatives. The closer the blood relationship, the more similar people should be.
Family studies alone don’t reveal whether a trait is genetically inherited. A family shares genes, but they also share similar environments. When researchers find trait similarity in a family study, their findings may suggest that the trait is genetically inherited, but the study can’t prove it.
Twin Studies
Compared to family studies, twin studies give researchers more solid evidence about whether a trait is inherited. In twin studies, researchers compare pairs of identical twins to fraternal, or nonidentical, twins. When doing these studies, researchers assume that identical twin pairs share the same environment, just as fraternal twin pairs do. However, identical twins share all of their genes with each other, while fraternal twins share only half of their genes. When a trait shows more similarity between identical twins than between fraternal twins, the greater similarity probably comes from shared genes, not shared environment.
One problem with this type of study is that identical twins may not in fact share an identical environment while fraternal twins do. People tend to treat identical twins in unusual ways. For example, people may treat identical twins as if they are similar in every respect, or they might focus intensely on differences between them.
Studies of Separated Twins
In order to avoid uncertain environmental factors, researchers sometimes study separated twins. Twins who are separated when they are very young and brought up in different families have different environmental influences, but identical genes. Trait similarities between separated twins result mostly from genes. However, separated twin studies can also be problematic. The environments of separated twins may not actually be that different from each other for the following reasons:
- The twins shared a similar prenatal environment before they were born.
- Adoption agencies may tend to place twins in similar households.
- Since they are similar in appearance and in genetically-inherited abilities, the twins may evoke similar responses from people around them.
As in other types of studies, trait similarities in separated twins may be due to both similar genes and similar environments.
Experiences and Behavior
Experiences affect behavior partly because environmental stimulation forms and maintains neural connections. For example, psychological research shows that babies need consistent, loving contact with a caregiver to achieve optimal brain development. Neglected babies who lack attention and physical contact, experience unpleasant emotions that are not simply transitory. Their experiences determine the development of their neural connections. Similarly, soldiers on active combat duty can suffer mental damage from the continuously stressful environment, even if they never experience physical injury.
Adoption Studies
In adoption studies, researchers compare adopted children to their biological parents and to their adoptive parents. Adopted children share more genes with their biological parents. The children’s living environments, however, more closely resemble the environments of their adoptive parents. When adoptive children resemble their biological parents more than their adoptive parents with respect to a certain trait, researchers can hypothesize that the trait has a genetic basis.
Interaction of Genes and Environment
In conducting these types of studies, researchers have found that while genes influence psychological traits, they don’t act alone. Highly influential environmental factors also play a major role. These factors include:
- Prenatal influences
- Child-rearing and other parental influences
- Nutrition
- Experiences throughout life
- Peer influences
- Culture
Cultural norms are sets of societal expectations that influence behavior. Norms tell us what kinds of behavior are appropriate. For example, in the United States, one cultural norm mandates that children be potty-trained by their third birthday. Parents of children who aren’t potty-trained by that point may start to feel worry, shame, and social pressure as their child’s third birthday passes.
Genes and environment interact in complex ways. People usually inherit a vulnerability or predisposition to having a particular psychological trait, and the environment in which those people live shapes the development of that trait. The opposite is also true: people’s psychological traits influence their environments. People don’t just live in environments—they also shape their worlds by exerting their traits.
Example: Suppose there are two nonidentical twins, Ben and Tom. Ben is calm by nature, while Tom has always been fussy. Mom and Dad will be more taxed by Tom, so they may be less responsive and patient with him than they are with Ben. Therefore, Tom and Ben experience different parental influences, which may make Tom less trusting than Ben as they grow up. Genes and environment influence Tom’s personality, but the interaction between genes and environment also plays a role.
