Unit 3 C Biological Bases of Behavior: Genetics, Evolutionary Psychology, and Behavior




Members of the human family share common behavioral tendencies but are also strikingly diverse. To what extent are we shaped by our heredity and to what degree by our life history? The conclusions— that nature is crucially important and that nurture is crucially important—are central to today’s psychology.

Genes provide the blueprints that design both our universal human attributes and our individual traits. Behavior geneticists explore individual differences. By using twin, adoption, and temperament studies, they assess the heritability of various traits and disorders. Their research indicates that both nature and nurture influence our life courses. We are products of interactions
between our genetic predispositions and our surrounding environments. Molecular geneticists search for genes that put people at risk for genetically influenced disorders, which has potential benefits as well as risks.

Evolutionary psychologists focus on what makes us alike as humans. They study how natural selection favored behavioral tendencies that contributed to the survival and spread of our genes. For example, in explaining gender differences in sexual behavior, they argue that women most often send their genes into the future by pairing wisely, men by pairing widely. Critics maintain that evolutionary psychologists make too many hindsight explanations and underestimate the role of culture.

The biopsychosocial approach to development recognizes that we are products of both nature and nurture, of genes and environment. We are also architects of our future. The stream of causation runs through our present choices.




Behavior Genetics: Predicting Individual Differences


Behavior geneticists study our differences and aim to determine the relative importance of heredity and environment on behavior. Environment includes every nongenetic influence, from prenatal nutrition to the people and things around us.
Every cell nucleus contains the genetic master code for the body. Within each cell are 46 chromosomes with 23 donated by each parent. Each chromosome is composed of a coiled chain of a molecule, called DNA (deoxyribonucleic acid). Genes are DNA segments that, when “turned on” (active or expressed), provide the code for the production of protein molecules. By directing the manufacture of proteins, the approximately 30,000 genes that compose the human body determine our physical development. The genome provides the complete instructions for making an organism, consisting of all the genetic material in the organism’s chromosomes. Variations at particular
gene sites in the DNA define each person’s uniqueness. Human traits are influenced by many genes interacting with the environment.

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Comparisons of identical twins, who are genetic clones, and fraternal twins, who develop from separate eggs, help behavior geneticists tease apart the effects of heredity and environment. On both extraversion and neuroticism, identical twins are much more similar than fraternal twins. The discovery that identical twins separated at birth show remarkable similarities also suggests genetic influence. Indeed, separated fraternal twins do not exhibit similarities comparable to those of separated identical twins. However, shared genes can translate into shared experiences.

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Adoption studies enable comparisons with both genetic and environmental relatives. Adoptees’ traits bear more similarities to their biological parents than to their caregiving adoptive parents. Nonetheless, the latter do influence their children’s attitudes, values, manners, faith, and politics. Clearly, nature and nurture shape one’s developing personality.

Heritability describes the extent to which variation among members of a group can be attributed to genes. If the heritability of intelligence is 50 percent, this does not mean that one’s intelligence is 50 percent genetic. Instead, it means that we can attribute to genetic influence 50 percent of the observed variation among people. Our genes affect how our environment reacts to and influences us. Nature enables nurture. Because of human adaptability, most psychologically interesting traits are expressed in particular environments. In other words, genes are self-regulating; they can react differently in different environments.

We are all the products of interactions between our genetic predispositions and our surrounding environments. For example, a stressful environment can trigger genes that affect the production of neurotransmitters that underlie depression. Breastfeeding boosts later intelligence only for the 90 percent of infants with a gene that assists in breaking down fatty acids present in human milk. Similarly, a baby who is genetically predisposed to be social and easygoing may, in contrast to one who is less so, attract more affectionate and stimulating care and thus develop into a warmer and more outgoing person.


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Molecular geneticists study the molecular structure and function of genes. They seek to identify specific genes influencing behavior. In labs worldwide, molecular geneticists are teaming with psychologists to identify genes that put people at risk for genetically influenced disorders. Potentially, steps may be taken to prevent problems before they happen. With this benefit, however, also comes risks of labeling people in ways that may lead to discrimination. Prenatal screening poses hopeful possibilities but also difficult problems as parents become able to select their children’s traits. In China and India, where boys are highly valued, testing for an offspring’s sex has enabled selective abortions. Millions of parents will select for health and perhaps for brains and beauty. However, by “selecting” out certain traits, we may deprive ourselves of future Handels, van Goghs, Lincolns, and Dickinsons, who were all troubled people.


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Evolutionary Psychology: Understanding Human Nature


Evolutionary psychologists focus on what makes us so much alike as humans. They study how natural selection has shaped our universal behavioral tendencies. Natural selection is the principle that, among the range of inherited trait variations, those that lead
to increased reproduction and survival will most likely be passed on to succeeding generations. Nature selects beneficial variations from among the mutations (random errors in gene replication) and the new gene combinations produced at each human conception. During human ancestry, genes that enable today’s capacity to learn and adapt had survival value. Similarly, we love the taste of fats and sweets, which once were hard to come by but which prepared our ancestors to survive famines. This particular natural disposition is mismatched with today’s junk-food environment.


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Gender refers to the biologically and socially influenced characteristics by which people define male and female. One of the largest reported gender differences is women’s greater disapproval of and lesser willingness to engage in casual, uncommitted sex. In comparison to women, men think more about sex, masturbate more often, are more likely to initiate sex, and make more sacrifices to gain sex.

Evolutionary psychologists apply the principle of natural selection to explain women’s more relational and men’s more recreational approaches to sex. Compared with eggs, sperm are cheap. While a woman usually incubates and nurses one infant at a time, a man can spread his genes by impregnating other females. Women most often send their genes into the future by pairing wisely,
men by pairing widely. Women increase their own and children’s chances of survival by searching for mates with economic resources and social status. Being attracted to healthy, fertile-appearing partners increases men’s chances of spreading their genes widely.

Critics argue that evolutionary psychologists start with an effect (e.g., gender sexuality difference) and work backward to propose an explanation. In addition, much of who we are is not hard-wired. Cultural expectations shape the genders. Still others suggest that evolutionary explanations may undercut moral responsibility. In response, evolutionary psychologists point to the explanatory
power of their theoretical principles, especially those offering testable predictions. They also note that understanding our propensities may help us to overcome them.


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Reflections on Nature and Nurture


Nature and nurture jointly form us. That is, we are products of natural selection and heredity as well as cultural, family, and peer influences. But we are also open systems—that is, creators as well as creatures of our worlds. We respond to the world’s response to us, and the stream of causation runs through our present choices. Our hopes, goals, and expectations influence our future. Our decisions today design our environments tomorrow.

Roger Sperry sees the mind and brain as a holistic system: The brain creates and controls the emergent mind, which in turn influences the brain.


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*Work Cited:  All summary notes come from *Myers Pyschology for AP, Lecture Guides (2011 Worth Publishers)