Temperament begins developing before birth and continues taking shape through early childhood. Researchers have detected measurable differences in fetal behavior as early as 24 weeks of gestation that correlate with childhood personality traits, and the core building blocks of temperament, including emotional reactivity, activity level, and attention, unfold in a predictable sequence over the first several years of life. Roughly 40 to 50 percent of temperament variation comes from genetics, with the rest shaped by environment and experience.
Signs of Temperament Appear in the Womb
Temperament doesn’t switch on at birth. Fetal movement patterns and heart rate measured during the second half of pregnancy already show meaningful links to later behavior. In a study of 333 mother-fetal pairs, fetuses with slower heart rates and less movement at 32 weeks of gestation were rated by their mothers as more behaviorally inhibited (shy, cautious, slow to warm up) in late childhood. Conversely, higher fetal motor activity between 24 and 36 weeks was associated with lower behavioral inhibition.
These prenatal signals are modest but real. How a fetus responds to stimulation also matters: at 24 weeks, fetuses that showed larger increases in movement when their mothers performed a stressful task were later rated as having fewer prosocial behaviors. Male fetuses appeared especially sensitive to these dynamics, with escalated heart rate or suppressed movement following maternal arousal predicting more behavioral problems in childhood compared to female fetuses with similar patterns.
The First Year: Reactivity Comes First
In the months after birth, the earliest and most visible dimension of temperament is reactivity, meaning how quickly and intensely a baby responds to stimulation. Newborns already differ in their sensory thresholds (how much sound, light, or touch it takes to get a reaction), their soothability, and their general activity level. These differences are present from the first days of life and form the raw material of temperament.
During the second half of the first year, temperament becomes more complex. Fear and wariness toward unfamiliar people or situations typically emerge around 6 to 8 months. Smiling and laughter increase after 6 months on a fairly steady upward trajectory. Interest, persistence, and the ability to shift attention all become more prominent during this same window. By the time a baby is approaching their first birthday, parents can usually identify a clear behavioral style: some babies are highly reactive to new experiences, others are calm and curious, and many fall somewhere in between.
Toddlerhood Through Preschool: Self-Regulation Emerges
If the first year is mostly about reactivity, the toddler and preschool years are when self-regulation starts to develop. This is the capacity to manage emotional and behavioral responses, and it depends heavily on maturing attention systems. A toddler who can redirect their focus away from something upsetting is using an early form of self-regulation, even if they don’t realize it.
The most significant regulatory capacity, called effortful control, develops primarily between ages 2 and 7. Effortful control is the ability to suppress a dominant response in favor of a less automatic one: waiting your turn, lowering your voice, stopping yourself from grabbing a toy. It requires the prefrontal cortex, the brain region responsible for planning and impulse control, which matures slowly throughout childhood and into the twenties. Children who develop strong effortful control tend to have better social skills, fewer behavioral problems, and an easier time in school.
The Brain Wiring Behind Temperament
The amygdala, a small structure deep in the brain that processes novelty and emotion, plays a central role in temperament from the start. Infants categorized as “high-reactive” (those who respond to new stimuli with vigorous limb movement and crying at four months) show measurably greater amygdala reactivity to unfamiliar faces almost two decades later. In one study, adults who had been high-reactive infants showed roughly 2.5 times more right amygdala activation when viewing novel faces compared to adults who had been low-reactive infants.
This doesn’t mean temperament is locked in by brain structure alone. The prefrontal cortex sends signals that can dampen amygdala reactivity, and this “top-down” regulation improves with age and experience. A child born with a highly reactive amygdala can learn to manage their responses over time, which is exactly what effortful control development looks like at the neural level.
How Genes and Environment Share the Work
Twin studies consistently find that genetics account for about 43 to 52 percent of variation in temperament traits like positive and negative emotionality. The remaining variance comes almost entirely from what researchers call the “nonshared environment,” meaning experiences unique to each individual rather than factors shared by siblings growing up in the same household. This is a striking finding: growing up in the same family doesn’t make siblings more alike in temperament, but their distinct experiences (different friendships, different classroom dynamics, different interactions with parents) do shape who they become.
The prenatal environment plays its own role through epigenetic mechanisms, which are chemical changes that affect how genes are expressed without altering the DNA itself. Maternal stress during pregnancy can change how a baby’s stress-response system is calibrated. In animal studies, prenatal stress leads to lasting changes in the brain’s stress hormone pathways, resulting in a more reactive stress system in offspring. Maternal nutrition matters too: low-protein diets during pregnancy increased anxiety-like behavior in male offspring in adulthood, while female offspring appeared to be somewhat protected, possibly due to differences in how genes in the amygdala were chemically modified.
Nine Traits That Define Temperament
The most widely used framework for understanding temperament in children comes from the New York Longitudinal Study, which identified nine distinct traits observable in infancy and early childhood:
- Activity level: how much physical movement a child shows throughout the day
- Biological rhythms: how predictable sleeping, eating, and other bodily functions are
- Sensitivity: how easily a child reacts to sensory input like noise, texture, or light
- Intensity of reaction: how strong the emotional response is, whether positive or negative
- Adaptability: how quickly a child adjusts to changes in routine or environment
- Approach or withdrawal: whether a child moves toward or pulls away from new situations
- Persistence: how long a child stays with a task when it becomes difficult
- Distractibility: how easily outside stimuli pull a child’s attention away
- Mood: the general tendency toward positive or negative emotional states
Not all of these traits appear at the same time. Activity level, sensitivity, and biological rhythms are observable within the first weeks of life. Approach or withdrawal and adaptability become clearer around 4 to 6 months as infants encounter more novelty. Persistence and distractibility become meaningful measures in the toddler years, once children are engaged in goal-directed activities.
Temperament Is Stable but Not Fixed
Behavioral inhibition, one of the most studied temperament traits, shows relative stability from toddlerhood through childhood. Children who are consistently inhibited are at greater risk for social withdrawal and anxiety disorders later in life. But stability is a tendency, not a guarantee. A child’s environment can push temperament in different directions over time.
The concept that matters most here is “goodness of fit,” the match between a child’s temperament and the demands of their environment. When parents, caregivers, and schools are responsive to a child’s natural behavioral style, development tends to go well. A highly sensitive child who is given time to warm up to new situations will follow a different trajectory than the same child who is pushed into overwhelming environments repeatedly. When that fit is right, even children with more challenging temperament profiles tend to thrive. When it’s poor, temperament traits that might otherwise be neutral can become the seeds of behavioral difficulties.
Temperament, in other words, is the starting point of personality, not the final product. The raw wiring is in place remarkably early, some of it detectable before birth. But the story of how that wiring gets shaped by experience, relationships, and the child’s own developing capacity for self-regulation continues well into adolescence and beyond.