Jerome Kagan is a developmental psychologist whose research focus was on temperament which is defined as being a pattern of personality traits and emotions that is generally stable over time. Kagan studied temperaments in children and believed that the environment wasn't the only influencing factor in the development of children. The behavioristic view that the environment was the only influencing factor on behavior was the prevailing theory at the time.
Kagan was interested in biological factors that are innate and can influence behavior. Kagan did long term research studies that followed individuals from infancy to adulthood. The results showed that anxious babies grew up to be anxious adults no matter what life situations and environment they were exposed to. This suggests that some personality traits like temperament come from biological factors which are not influenced by the environment. The biological factors at play influence how vulnerable a child is to emotions.
Kagan believed that biology and environmental factors were both important in the development of personality and behaviors.