Throughout this chapter we have discussed the formative role of the environment in the development of an infant. However, there is also a body of research that looks at infant characteristics that may be inborn and not learned, a product of nature rather than nurture. Such characteristics are known as temperament. In 1956, Alexander Thomas and Stella Chess initiated a decades-long study of temperament. This study followed infants throughout childhood and adolescence into early adulthood. They identified nine dimensions of temperament related to activity level and response to stimulation and stress.

These nine traits include: activity level, rhythmicity, approach/withdrawal, adaptability, attention span and persistence, intensity of reaction, threshold of responsiveness, and quality of mood. More recently, Mary Rothbart simplified Thomas and Chess’s definition of temperament into two general categories, reactivity and self-regulation. Her specific temperament dimensions included activity level, smiling and laughter, fear, distress to limitations, soothability, and duration of orienting.