In psychology, temperament refers to biologically rooted tendencies — like reactivity to stimulation — that are largely present from birth. Harvard psychologist Jerome Kagan famously showed that infants just a few months old could be sorted into "high-reactive" and "low-reactive" groups based on how intensely they responded to unfamiliar stimuli, a pattern that tended to track with later shyness or boldness.
Temperament is closer to the raw material of personality. Two people born with the same temperament can develop into very different personalities depending on the environment they grow up in.
Personality is a far more complex result — inborn temperament shaped over time by parenting style, cultural background, relationships, and the accumulation of life events large and small. A child born highly reactive to stimulation, for instance, often grows into a thoughtful, careful adult rather than an anxious one, given a stable and supportive upbringing.
Behavioral genetics studies comparing identical and fraternal twins raised apart generally report that personality traits are roughly 40–60% heritable. The rest is attributed to environmental factors — family environment, peer relationships, and experiences unique to each individual.
This roughly matches the popular idea that personality is "half nature, half nurture," though the more accurate picture is that genes and environment don't act independently — they interact with each other in complex ways.
Long-term studies show personality traits aren't fixed for life — people tend to gradually become more conscientious and emotionally stable with age, a pattern known as the maturity principle. Inborn temperament may be slow to shift, but the personality built on top of it can keep being reshaped throughout life through conscious effort and new experience.