9.3.7 - Nature and nurture

What nature and nurture explain

Nature and nurture refer to different sources of influence, rather than two separate types of person.

Genes can influence the development and functioning of biological systems; maturation can change what a person can do as those systems develop. Nurture includes more than deliberate parenting: nutrition, illness exposure, education, peer relationships and opportunities to practise can all matter. A biological process is not necessarily inherited. Brain activity can change with learning or injury, so detecting a brain difference does not establish that it was present at birth.

An extreme nativist explanation gives primary importance to inherited characteristics. An extreme empiricist explanation gives primary importance to experience. In psychology these positions generate testable questions, but neither label is itself evidence. Saying that aggression is 'natural' does not identify a gene or demonstrate how it affects behaviour; saying it is 'learned' does not identify the experiences that produced it.

For memory, development of neural systems may support increasing capacity, while language experience and strategies affect performance on a particular task. For obedience, an ability to respond socially requires biological capacities, but who is accepted as an authority and which requests seem legitimate can be learned. A complete account must distinguish the capacities that make behaviour possible from influences explaining why it occurs here and now.

The debate also concerns differences between people. Discovering that all people require a functioning brain tells us little about why two people perform differently. Research must examine relevant variation, use an appropriate comparison and avoid assuming that the answer to one question settles every nature–nurture question.

What genetically informed evidence shows

Twin and adoption comparisons help investigate inherited influences, but their conclusions depend on environmental and measurement assumptions.

Monozygotic twins share nearly all their inherited DNA sequence; dizygotic twins share, on average, about half the genetic variants that vary between people. If monozygotic pairs are more similar on a measured behaviour, this is consistent with genetic influence. The comparison is strongest when environmental similarity relevant to that behaviour is not systematically greater for the monozygotic pairs. Being treated more similarly could otherwise contribute to the pattern. Twins are not randomly assigned to their genetic or social circumstances.

Brendgen and colleagues' study assessed physical and social aggression in six-year-old twins using teacher and peer reports. Its models found genetic and nonshared environmental contributions to physical aggression, with stronger environmental contributions and weaker genetic effects for social aggression. This supports a differentiated account rather than a claim that all aggression has one source. Agreement across informants strengthens the pattern, but ratings and model assumptions still affect the estimates. The study did not identify a gene that inevitably produces an aggressive act.

Heritability is an estimate of how much variation within a studied population is associated with genetic differences under the model and conditions used. It is not the proportion of one person's behaviour 'caused by genes'. An estimate can change when environments or measurements change. High heritability therefore does not mean that an intervention cannot help.

Adoption research separates biological relatedness from the family doing the rearing more clearly than a simple family comparison. It still needs to consider prenatal influences, selective placement and differences between adoptive homes. A resemblance to biological relatives supports an inherited contribution only after plausible alternative explanations have been considered.

Raine's PET comparison found brain-functioning differences between people pleading not guilty by reason of insanity and matched controls. It supports investigating biological mechanisms, but brain functioning is not genetic evidence by itself. A retrospective association cannot establish that an inherited abnormality caused violence.

Evidence that experience changes behaviour

Learning and social research can demonstrate environmental influence without showing that biology makes no contribution.