The terms "easy" and "difficult" applied to children's temperament come from Alexander Thomas and Stella Chess's longitudinal New York Longitudinal Study, begun in 1956 — the most important single study in the history of temperament research. Thomas and Chess tracked 133 children from infancy through adulthood, coding their behavioural profiles along nine dimensions and identifying three temperament clusters that have shaped how developmental psychologists and paediatricians talk about individual differences in children ever since. Understanding what these clusters actually mean — and what the research since Thomas and Chess says about their implications — is useful for anyone raising, teaching, or working closely with children.
The Nine Dimensions Thomas and Chess Identified
The clusters were built from ratings on nine temperamental dimensions observed in infancy and early childhood:
- Activity level: How much the child moves — wriggly in the bath, running versus walking, restless during sitting
- Rhythmicity: Regularity of biological functions — sleep, hunger, bowel patterns
- Approach/withdrawal: First response to new people, foods, or situations — approaching with curiosity versus pulling back
- Adaptability: How readily the child adjusts after initial reaction — whether a new situation becomes comfortable quickly or remains uncomfortable
- Threshold of responsiveness: How much stimulation is needed to produce a reaction — the child who wakes to any noise versus the one who sleeps through a party
- Intensity of reactions: How strong the emotional response is to positive or negative stimuli — the child who cries briefly versus the one who screams for twenty minutes
- Quality of mood: Baseline positive versus negative affect — cheerful versus serious or fretful as a default
- Distractibility: How easily the child is diverted from what they're doing
- Attention span and persistence: How long the child stays with a task, including in the face of frustration
The Three Clusters: Easy, Difficult, and Slow-to-Warm-Up
From these nine dimensions, Thomas and Chess identified three main clusters — which they estimated characterised roughly 65% of their sample (the remaining 35% showed mixed profiles).
The Easy Child (approximately 40% of the sample)
Regular biological rhythms, positive approach to new situations, high adaptability, low to moderate intensity, generally positive mood. These children move relatively smoothly through developmental transitions, take to new people and situations with curiosity rather than alarm, and don't demand a great deal of regulatory scaffolding from caregivers. Thomas and Chess noted that the "easy" label can be misleading — these children require attentive parenting, but they create fewer day-to-day management challenges.
The Difficult Child (approximately 10% of the sample)
Irregular biological rhythms, withdrawal from novelty, slow adaptability, high intensity, predominantly negative mood. These children are harder to settle into routines, respond to new situations with distress rather than curiosity, take longer to adapt, and when they do react — positively or negatively — they react strongly. Thomas and Chess were careful to emphasise that "difficult" referred to demands on caregivers, not to the child's prognosis. The temperament cluster that's difficult in the context of average parenting can be managed well with responsive, patient care, and some of the same traits (intensity, persistence, strong reactions) that cause difficulty in childhood can become assets in adulthood.
The Slow-to-Warm-Up Child (approximately 15% of the sample)
Low activity level, withdrawal from novelty, slow adaptability, low intensity, mild-to-negative mood. Similar to difficult in the withdrawal pattern but much quieter in expression. These children don't dramatically protest new situations — they simply hang back, watching, before gradually moving in. With patience from caregivers, they eventually warm up and engage fully; the problem arises when adults or environments push too fast for the slow-to-warm-up child's pace, at which point avoidance deepens.
Goodness of Fit: Why the Label Isn't the Point
Thomas and Chess's most enduring contribution wasn't the three clusters — it was the concept of "goodness of fit." The outcome for a child isn't determined by temperament alone; it's determined by how well the environment, and particularly caregivers, match their approach to the child's temperament.
A highly active, intense child raised by patient, low-reactive parents in an environment that allows physical movement will likely do well. The same child raised by anxious, easily overwhelmed parents in a restrictive environment will likely develop behaviour problems — not because of their temperament but because of the mismatch. Conversely, a slow-to-warm-up child whose caregiver patiently allows them their adjustment time will eventually engage fully; the same child pushed prematurely into situations will withdraw further.
This reframe shifted the clinical conversation from "how do we fix this child's temperament" to "how do we adjust the environment to fit what this child needs." It's the foundation of most modern temperament-informed parenting guidance.
What the Research Since Thomas and Chess Has Added
Fifty years of follow-up research has broadly supported Thomas and Chess's framework while refining it:
- Stability: Temperament dimensions show moderate stability from infancy through adolescence, particularly activity level, reactivity, and approach/withdrawal. They're not fixed — experience, parenting, and deliberate development can shift them — but the baseline is relatively stable compared to mood or behaviour.
- Neural correlates: Jerome Kagan's work on behavioural inhibition (closely related to the withdrawal dimension) identified specific physiological markers — higher startle responses, elevated heart rate to mild stimuli, heightened amygdala reactivity — that underlie the inhibited temperament pattern.
- The Big Five connection: Adult personality traits map reasonably onto childhood temperament dimensions. High reactivity and negative mood correlates with adult neuroticism; approach orientation and positive mood correlates with extraversion; persistence and low distractibility correlates with conscientiousness.
- Differential susceptibility: Jay Belsky's "difficult" children aren't just more vulnerable to bad environments — they're also more responsive to good ones. High-reactive children placed in high-quality parenting environments tend to do better than easy children in those same environments. The same sensitivity that creates vulnerability also creates enhanced benefit from positive input.
Practical Implications for Parents and Teachers
For parents of high-intensity, low-rhythmicity, slow-to-adapt children:
- Predictable routines reduce the regulatory load significantly — the difficult child's biology works against regularity, so the environment needs to supply what the child's rhythms don't
- Previewing transitions ("in five minutes we're leaving the park") reduces the intensity of transition protests more than abrupt endings
- Labelling the child as "difficult" to others — or internally — shapes how interactions go; "intense" and "sensitive" describe the same traits without the deficit framing
For teachers managing slow-to-warm-up children:
- Allow observation time before expecting participation — pushing for engagement before the child is ready increases avoidance
- Pairing new activities with familiar children or contexts lowers the novelty cost
- Progress on engagement timeline is real progress even when it's slower than peers
For those interested in how adult personality dimensions trace back to childhood temperament, our free Big Five personality test maps the dimensions that most closely track from childhood temperament into adult trait expression.
Frequently Asked Questions
Is "difficult" temperament a diagnosis?
No. Thomas and Chess used the term descriptively to characterise a profile that creates demands on caregivers — not as a diagnostic category or a prediction of disorder. A child with a difficult temperament profile can develop entirely typically with appropriate caregiving. The profile is associated with slightly elevated risk for behaviour problems when goodness of fit is poor, but this is a probabilistic statement, not a prognosis.
Can a child's temperament change over time?
Somewhat. The underlying dimensions show moderate stability, but they're not fixed. Parenting quality, peer experience, and the child's own developing self-regulation all influence how temperament expresses over time. High-reactive children often develop effective coping strategies that reduce the behavioural expression of reactivity even if the underlying sensitivity remains. The trait tends to become more manageable with maturation.
How early can temperament be identified?
Reliably by about four months. Some dimensions — activity level, reactivity — are observable in the first weeks, but consistent profiles emerge more clearly as the nervous system matures. The behavioural inhibition pattern (slow-to-warm-up tendency) is reliably identifiable by the first birthday in research settings.
Are "easy" children actually easier to raise?
In terms of day-to-day management, generally yes. But Thomas and Chess noted that easy children can be overestimated — parents may not notice problems developing because the child isn't protesting. An easy child adapting too readily to environments that aren't actually good for them can create a different kind of parenting challenge. Absence of complaint isn't the same as absence of need.
Does difficult temperament predict adult outcomes?
Only modestly, and the prediction is strongly moderated by parenting quality and goodness of fit. Thomas and Chess's long-term follow-up found that difficult children who received responsive parenting did not show elevated rates of adult problems. The stronger predictor of adult outcomes was the match between temperament and environment during development, not temperament alone.