Long before personality forms, babies already differ in how strongly they react to the world and how well they can settle themselves. That early wiring tilts a whole life, without ever fixing it in place.
Ask four temperament researchers to define the term and you used to get four answers, which is exactly what a famous 1987 roundtable found (Goldsmith et al., 1987). Twenty-five years on they had converged on a workable core (Shiner et al., 2012): temperament is the set of early-appearing, biologically based individual differences in reactivity and self-regulation. Two words carry it. Reactivity is how strongly and quickly you respond to what happens, the loudness of the startle, the speed of the upset, the pull toward something new. Self-regulation is how well you can turn that volume down. These show up in infancy, before culture and experience have done much, which is what marks temperament off from the fuller personality it later grows into (see L2-01).
Measure it carefully and the differences organise into three broad dimensions (Rothbart, Ahadi, Hershey et al., 2001). Surgency, roughly early extraversion: activity, positive excitement, approach. Negative affectivity: proneness to fear, frustration, and distress. And effortful control: the budding ability to focus attention and check an impulse, the seed of self-regulation that later feeds into executive function and conscientiousness. Strikingly, this three-part structure turns up not just in Western children but in samples from China and Japan, which makes it travel better across cultures than the adult Big Five managed to.
The most studied single temperament is behavioural inhibition, mapped by Jerome Kagan. Some children meet anything unfamiliar, a stranger, a new room, an odd toy, with caution and restraint, while others barrel in. Kagan, Reznick and Snidman (1988) followed such children and found the restraint was both stable and physical: the wary two-year-olds were the quiet, socially avoidant seven-year-olds, and they showed heightened bodily arousal, later traced to a more reactive amygdala. About one in seven children sits at the strongly inhibited end.
And temperament has real reach into later life, though a measured amount. In the Dunedin study, a thousand three-year-olds were sorted into five temperament styles and then re-examined at 26; the early styles were still legible in their adult personalities more than two decades later (Caspi et al., 2003). The clinical signal is sharper for inhibition: a meta-analysis found that behaviourally inhibited children had roughly seven and a half times the odds of developing social anxiety disorder (Clauss and Blackford, 2012). The reactivity you are born with is not just a phase.
The biggest caution is that temperament is a tilt, not a destiny. That same meta-analysis is usually quoted for the alarming seven-fold figure, but its other half matters just as much: only about half of even strongly inhibited children go on to an anxiety disorder, which means about half do not. What separates the two paths is largely the environment, and specifically the goodness of fit between a child's temperament and the demands around them, an idea from the original New York Longitudinal Study (Thomas and Chess, 1977). A cautious, slow-to-warm child can do well with patient, predictable handling and struggle with a pushy, chaotic one. The trait sets the starting odds; the environment moves them.
Two more caveats. Most temperament data, especially the questionnaires, come from parents rating their own children, which imports the usual problems of projection and halo (a fuller treatment in L2-08). And in adults the line between "temperament" and "personality" is more semantic than real: the three dimensions map straight onto the Big Five's extraversion, neuroticism, and conscientiousness, so temperament is best seen as the early, biological end of the same continuum, not a separate thing.
Predictions from infancy to adulthood are modest, with genuine plasticity along the way, so any single early reading is a weak forecast of one later outcome. The reliance on parent report is hard to escape at the youngest ages. And the label "difficult temperament" is loaded: what counts as difficult depends on what a particular family, school, or culture is set up to tolerate, so the word describes a relationship, not just a child.
Can we measure temperament cleanly, without leaning on a parent's eye or on the very behaviour we are trying to predict? Why do some early tendencies harden while others wash out? And how durable are interventions that improve the fit between a child and their environment, over years rather than months?
Read temperament as a starting propensity in reactivity and self-regulation, not a fixed fate, and pay as much attention to the fit between a person and their setting as to the trait itself.
Temperament mostly reaches your customers in adult form, as stable differences in threat sensitivity and the appetite for novelty. People high in negative affectivity gravitate toward what calms a worry (insurance, warranties, security), while those high in surgency and novelty-seeking are drawn to what promises stimulation (adventure travel, new launches, variety). Effortful control shows up wherever self-regulation is the product, in saving, dieting, and sticking with a plan. Two honest limits: these are mild propensities, not segments to bet the brand on, and adult self-report of them carries all the wobble covered in L2-08, so treat any temperament-based targeting as a gentle prior, not a profile.
There is a real, if small, thread linking the negative-affectivity and threat-sensitivity end of temperament to more security-focused political attitudes, which is part of why fear-based messages land differently on different people (this connects to the cultural and risk-perception material in L1-10). It is a tendency worth understanding and a poor basis for stereotyping anyone.
Behavioural inhibition is one of the largest early, identifiable risk factors for anxiety disorders (Clauss and Blackford, 2012), which makes early childhood a place where prevention can do real good. But the lever is fit, not labelling. Because roughly half of inhibited children never develop a disorder, the useful move is to support environments that suit cautious temperaments (warm, predictable, gently encouraging) rather than to tag children as problems. The same logic should make services wary of the phrase "difficult child", which often names a mismatch the system could fix.
Three habits. First, treat temperament as a propensity you are reading, not a sentence you are passing: it shifts the odds, it does not set them. Second, look at the fit, because the same reactive wiring thrives in one environment and suffers in another, and the environment is usually the more changeable half. Third, remember that in adults temperament has blended into personality, so the cautions from the personality piece (it describes, it does not explain; mind the measurement) apply here too.
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