It is a rich, interdisciplinary field of research that explores the two-way relationship of biology and psychology. These two tend to affect one another; for instance, our emotions can affect the chemicals our bodies produce. If sports' fans witness their favorite team losing, it apparently lowers their levels of testosterone. In the other direction, a woman going through her period is renowned to have fluctuating emotions.
Before the invention of brain-scanning techniques, attempts to study the brain and its biology were slightly shoddy. The only way we could make assumptions about what part of the brain affects what behavior is through the autopsies of people with 'damaged brains' or special needs. Another field that was misguided in its attempts to understand the brain would be phrenology; it was believed that since the brain worked out like a muscle, certain parts would 'swell' and be noticably bumpy, suggesting things about a person.