Innate and acquired influences on behaviour

Interaction means that the effect of a genetic endowment depends on the environment, and the effect of an environment depends on the endowment. The clearest formal model is the range of reaction: a genotype does not fix one outcome but a range of possible outcomes, and the environment determines where inside that range the person develops. Phenylketonuria shows the mechanism clinically. A child who inherits two faulty copies of the PAH gene cannot metabolise phenylalanine, and untreated this causes severe intellectual disability; detected at birth and managed with a controlled diet, the same genotype produces normal development. Nothing about the gene changed — only the environment in which it acted. Sandra Scarr and Kathleen McCartney added three pathways of gene–environment correlation. In a passive pathway the family supplies both the genes and the environment, as when musical parents provide both an endowment and a house full of instruments. In an evocative pathway the child’s inherited disposition draws particular reactions from others, so a highly irritable infant may be handled less gently. In an active pathway the growing person selects environments that match his disposition. Because genes and environments are correlated, the old question ‘how much is genetic?’ is poorly posed. Heritability is the statistic used to answer it, and it must be read precisely: it is the proportion of the measured variation between individuals in one population at one time that is associated with genetic differences. It is a property of a group, not of a person, and a high value does not mean a trait is unchangeable. Adult height is strongly heritable, yet average height rose dramatically across the twentieth century as nutrition and childhood infection improved — a change produced entirely by environment in a highly heritable trait. A nurse who reads heritability as destiny will stop looking for the conditions that are still modifiable.

A sensitive period is a window in development during which a system is especially responsive to experience, so that the same event has a much larger and more lasting effect than it would later. The classic animal evidence comes from Konrad Lorenz, who showed that greylag goslings follow the first moving object they see in the hours after hatching — imprinting that does not occur if the exposure is delayed. David Hubel and Torsten Wiesel later showed that closing one eye of a kitten for a few weeks permanently altered the organisation of its visual cortex, while the same deprivation in an adult cat had little effect. In humans, John Bowlby argued that the first years are when the child builds an internal working model of attachment from the consistency of care, and the Bucharest Early Intervention Project — a randomised trial in which institutionalised Romanian children were assigned to foster care at different ages — found that the earlier the placement, the better the cognitive and social outcomes, although placement still helped at every age studied. Language is the clearest everyday example: a child who hears no language before adolescence will not acquire grammar normally, while an adult can still learn vocabulary. The practical implication is precise and limited. A sensitive period is not a deadline after which nothing can change; it is a period when support produces the largest return. It names the mechanisms behind the first-thousand-days message in maternal and child health: folate and iodine before and during pregnancy, exclusive breastfeeding, kangaroo mother care for the preterm infant, immunisation and growth monitoring, and responsive stimulation in the first two years. It also explains why late detection is expensive. A child whose hearing loss is found at six months and fitted with a hearing aid enters school with language; the same child detected at five often does not. Screening, early referral and follow-up are therefore not administrative extras — they are the intervention itself.

Epigenetics is the study of chemical marks that change how a gene is used without changing the DNA sequence. Three mechanisms carry most of the effect: methylation, in which a methyl group attaches to cytosine bases and usually silences the gene; histone modification, in which proteins around which DNA is wound are chemically tagged, opening or closing the region; and non-coding RNA, small molecules that regulate gene expression after transcription. The marks are real and measurable, and they give the innate/acquired debate a physical mechanism. In the agouti viable yellow mouse, the offspring of mothers fed a diet rich in methyl donors — folate, vitamin B12, choline and betaine — differed from their genetically identical siblings in coat colour and later body weight, because the diet had changed methylation at a specific regulatory region. In humans, the Dutch Hunger Winter of 1944–1945 produced a natural experiment: people exposed to famine in early gestation were found six decades later to carry altered methylation at the IGF2 gene, and the effect depended on the timing of exposure, with early gestation differing from late. In rats, Michael Meaney’s group showed that pups whose mothers licked and groomed them more grew into adults with a calmer stress response, mediated by methylation of the glucocorticoid receptor gene NR3C1 and by the amount of receptor in the hippocampus. Three cautions are essential before this is applied to patients. Most human epigenetic findings are observational, effect sizes are usually small, and methylation patterns are tissue-specific, so a blood sample does not describe the brain. Epigenetic marks are also not simply inherited across generations in the way DNA is; transgenerational claims remain contested. The safe nursing message is narrow but important: nutrition, chronic stress and the quality of early care are biologically active, not merely psychological background. This is a reason to take antenatal nutrition, maternal mental health and infant care seriously as clinical work.

Enculturation is largely finished before a person can examine it, which is why the processes that come after it need to be named precisely. Acculturation is defined by Robert Redfield, Ralph Linton and Melville Herskovits in 1936 as the change that follows continuous firsthand contact between groups of different cultures. It is not one-way and not automatic, and it is often selective: a family may adopt hospital birth and clinic vaccination while keeping its food rules, its language and its healer. John Berry’s framework organises the outcomes by asking two questions — is it important to maintain one’s heritage culture, and is it important to participate in the larger society? The answers define four strategies: integration, assimilation, separation and marginalisation. Integration, sometimes called biculturalism, is generally associated with the best psychological adaptation and marginalisation with the worst, though the pattern depends heavily on how welcoming the receiving society is, and perceived discrimination often predicts distress better than the strategy itself. Acculturative stress is the strain of that change: language barriers, changed diet, the loss of the extended family’s practical help and of the traditional healer’s recognised role. Assimilation is one outcome among four, not the destination of every migrant and not a measure of successful care. For nursing, the practical rule is to ask which strategy the family is actually following rather than assuming one, and never to read the survival of a traditional practice as a refusal to adapt.

A claim that something is ‘just human nature’ has a testable form and an untestable use. In its testable form it predicts that the behaviour appears across different conditions and different communities, and that removing the learning opportunity will not remove the behaviour. To test it you need three things: an operationally defined behaviour that can be observed and counted, real variation in the conditions that are supposed to produce it, and a prediction that some observation could prove wrong. The everyday use does the opposite. It takes one local pattern — men in this town do not cry in public, mothers here sleep beside the newborn — and elevates it to a universal, which makes the claim unfalsifiable and hides exactly the variation that would settle it. Two errors then follow. The first is the is–ought problem stated by David Hume: showing that something is the case does not show that it ought to be the case (G. E. Moore later named the related move in ethics the naturalistic fallacy), so a fact about human nature can never by itself justify a rule of care. The second is the misreading of heritability as immutability, which we met earlier. Margaret Mead’s account of adolescence in Samoa was for decades the standard evidence that adolescent turmoil is cultural rather than biological; Derek Freeman’s 1983 critique of her methods and conclusions showed that even a celebrated finding is revisable, which is a reason for caution rather than for cynicism. A nurse can apply the test in one minute. Ask what would be observed if the behaviour were learned rather than innate — who in the same community behaves differently, and under what conditions? If no possible observation would count against the claim, it is not an explanation; it is a word used to stop the enquiry.