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The nature/nurture controversy has been one of the more enduring themes of psychological research. Although it used to be an either/or question of heredity or environment, now most psychologists agree with an interactionist point of view.

Jerome Kagan did some interesting longitudinal research with infants. By 3 months of age, 15–20% of children in his sample were already expressing inhibited behavior in the form of startle reactions to new stimuli. In their teens, 75% of his original group of inhibited kids were still inhibited. American parents who seem to value more extraverted behaviors were unable to change their children’s nature. Strong correlations have been found between inhibited children and parents, and grandparents as well.

Many people seem to learn shyness from abusive parenting styles, negative experiences in school, and rejection in social situations as both children and adults. Some 50% of Americans self-report shyness. Philip Zimbardo has spent much of his life conducting research and therapy for shyness. Cognitive-behavioral therapy has been successful in helping formerly shy people to learn new social skills and in reinforcing more positive self-statements.

Language acquisition has also had its nature and nurture supporters. Prominent on the nature side is linguist Noam Chomsky, who argues for a language acquisition device in the brain. He cites evidence of a progressive sequence of language acquisition from cooing, babbling, holophrases, and telegraphic speech to even overgeneralization of grammar rules unaffected by learning. Chomsky has indicated a critical period during which a child must be exposed to language for this maturational process to occur. Although B. F. Skinner would agree on the steps in the language acquisition process, he disagrees with Chomsky and believes that language is acquired through shaping and reinforcement. Although all children babble some 100 phonemes at about 6 months of age, 10 at months most are only using the phonemes of their own language, 40 some in English that have been reinforced.

Phenylketonuria or PKU is an inherited problem of metabolism. In order to be expressed, two recessive alleles for this trait must be present. Today, screening for this metabolic disorder is done at birth and the mental retardation and other problems associated with it in the past have been considerably altered. Following a restricted diet, at least until adolescence, can prevent expression of phenylketonuria. The diet eliminates proteins, nuts, and dairy products and severely restricts starches like bread and potatoes.

There are many theories on both the nature and nurture side about violent behavior. Both Freud and Lorenz believed that aggression is innate and the frustration–aggression hypothesis claims that violent behavior is a natural product of built-up frustration. High testosterone levels and lower serotonin levels have also supported a biological basis for violent behavior. Some studies have indicated that children more closely mimic their violent biological parents than their adopted parents. On the nurture side, Bandura’s social learning theory and Bobo doll studies indicate that violent behavior can be learned through modeling and imitation. Recent studies indicate that children who play violent video games are more likely to display violent behavior than those who do not play such games. Gangs who commit violent crimes reward similar behavior by their members and punish those who do not conform to these norms. Finally, in a highly aroused state, many people fall into a mob violence situation, as soccer matches and social protest marches have shown, exemplifying deindividuation.

Some psychologists also think the causes of schizophrenia are biological, whereas others think its causes are environmental. Schizophrenia is a devastating mental disorder involving delusional thinking, sensory hallucinations, and other psychotic symptoms. High levels of dopamine that can be treated with antipsychotic drugs in many cases seem to support the biological origin of this disease.