What explains intelligence? Nature? Nurture? Both?

Both. In a twin study conducted in 2003, the degree to which genes affected intelligence differed at the different socioeconomic status (SES) levels. Among participants in high-SES families, genetics explained most of the variations in intelligence, but among poor families, genetics explained hardly anything at all. The influence of genes depended upon the environment.

What can lactose intolerance tell us about whether biology or culture came first?

Cultural differences created variations in “lactose tolerance” genes. If milk was a major food source in a given culture, then, and only then, did most people develop the ability to digest lactose, because those who had the ability to digest lactose were more likely to survive to reproduce.

The Cultural Opportunities feature describes examples of how biology can shape culture (e.g., higher levels of pathogens are associated with greater collectivism) and how culture can shape biology (e.g., the cultural practice of marrying within one’s group is associated with decreased genetic diversity).

What does it mean to say that nature is dependent on nurture?

Nature and nurture work together to produce the psychological characteristics that make us human. For example, according to research on genes, intelligence, and poverty, in some environments most of the differences between people are accounted for by genes. In other settings, environmental factors account for most of the differences.

How can identical and fraternal twins’ data be used to tell us about the influence of genes?

In the twin method, researchers compare the psychological similarity of monozygotic (identical; MZ) twins and dizygotic (fraternal; DZ) twins. If MZ twins resemble one another more closely than do DZ twins, the cause of the greater resemblance must be genetic because the only difference between the two types of twins, presumably, is their genetic similarity (although recent research, described in This Just In, indicates that not all identical twins are genetically identical!).

What do twin studies tell us about the heritability of personality, psychological disorders, and intelligence?

Twin studies commonly find that identical twins are more similar, psychologically, than are fraternal twins and that heritability depends on the quality being measured. Schizophrenia, for instance, is highly heritable, whereas social and political attitudes are lower in heritability.

What false conclusions should we avoid making when interpreting the results of twin studies?

First, do not assume that the heritability of psychological characteristics indicates those traits cannot change. Second, 50% heritability for a trait does not mean that half of each individual’s trait is inherited. Third, high heritability within a group does not mean that differences between that group and some other group must be due to genetics.

What is the relation between chromosomes, DNA, genes, and the genome?

Genetic information is contained in chromosomes; a critical molecule found in the chromosome is deoxyribonucleic acid, or DNA, which contains the instructions for building an organism. These instructions are encoded in sequences of molecules that contain instructions for building protein molecules. A gene is a stretch of DNA that provides the full information needed to produce a protein. The genome is the entire set of genes for an organism.

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How do we get from DNA to protein building?

In the process known as gene expression, a molecule in the cell reads the information in DNA and creates another molecule called ribonucleic acid, or RNA. The RNA, which contains the information needed for protein building, moves to a mechanism in the cell that contains the biochemical machinery needed to build those proteins. Once there, cellular machinery reads the information encoded in RNA, gathers the molecules needed to build a protein, and assembles them into a protein.

How do our experiences affect gene expression? In other words, how does nurture influence nature?

Environmental experiences “switch on” genes, causing them to become active in the production of proteins. For example, in one study, researchers found that people with one type of genetic makeup rarely contemplated suicide, no matter how many stressful events they experienced. However, among people genetically predisposed to experience depression, those who suffered a large number of stressful life events were far more likely than others to consider suicide.

How do characteristics—psychological and otherwise—evolve?

Nature selects them. In any naturally occurring environment, some features possessed by an organism are more adaptive than others, thereby helping organisms to survive and reproduce in that environment. Across generations, these features become more frequent because the reproducing organisms pass them down to their offspring.

According to evolutionary psychologists, how did the mind evolve to produce our contemporary mental abilities, desires, and preferences?

Some evolutionary psychologists propose that the human mind acquired mental modules: elements of the mind and nervous system that perform a specific task in response to a specific type of environmental input. According to this concept, elements of mind that evolved in the evolutionary past shape our present preferences (e.g., a male’s preference for women with wide hips is a preference that evolved because it increased the odds of successfully reproducing).

What are three limitations of evolutionary explanations of the modern mind?

First, subsequent research findings failed to confirm some original findings. Second, much human mental activity cannot be explained by mental modules alone. Third, genes do not, by themselves, determine the nature of an organism; rather, organisms develop through dynamic interactions between nature and nurture.

Can evolutionary theory explain homosexuality?

Some scholars have suggested that homosexuality itself may not have evolved. Instead, what evolved is a particular gene that has different effects for men and women. In men, the gene increases the likelihood of homosexuality, whereas in women, it increases the likelihood of childbirth.

What does research suggest concerning the genetic basis of homosexuality?

Twin studies indicate that genetics explains slightly more than one-third of the overall variability in sexual orientation, whereas shared environmental factors have no effect at all. Prenatal factors may influence sexual orientation. Men who have larger numbers of older brothers are more likely to be homosexual, possibly because the mother’s immune system recognizes a male child as biologically “foreign” (because she is female) and produces antibodies that affect fetal brain development in a way that increases the likelihood of homosexuality.

How do genes shape brain development?

Genetic information causes neurons to connect from one general region of the brain to another; however, these region-to-region connections are not precise enough for the brain to function properly. The brain requires experience to form the exact neuron-to-neuron connections.

How does environmental experience shape brain development?

Environmental experience influences the connections among brain cells. When environmental stimulation causes a group of neurons to fire at the same time, the interconnections among those neurons grow stronger. Connections among neurons that don’t fire together grow weaker, and through neural pruning, these unneeded connections are eliminated. Environmental experience also affects neural connections by influencing gene expression.