Although the field of psychology has existed for over a century, it was long considered a soft science. The hard sciences were those that could precisely measure objects and events within the physical world. For example, physicists know that an atom of iron is 55.405 times heaver than an atom of hydrogen, and biologists have now mapped out the human genome.

Although psychologists could observe and measure the external elements of behavior, they couldn't observe the internal biological processes that led to the behavior. Further, many of our early beliefs about human psychology came from introspection, which doesn't report what actually occurred within the subject's brain.

Recent dramatic advances in brain imaging technology have led to the observation and measurement of activity in specific brain systems, so the field of psychology (now typically called cognitive neuroscience) is finally able to explore the biological antecedents of decision and behavior.