What area of the lifespan did scientists first study?

The correct answer is child development.

Explanation:

A scientific study of how and why human beings transform over the course of their life is known as developmental psychology. It was initially concerned with children and infants, however, the field enlarged to comprise adult development, adolescence, aging, and the whole lifespan.  

In the area of the lifespan, the scientists initially studied the development of a child. The child development covers the psychological, biological, and emotional modifications, which take place in human beings between the birth and the end of adolescence.  

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