Expectancy value theory was proposed by Martin Fishbein in the 1970s. According to the theory, a person’s behaviour is determined by how highly a goal is valued, and by the degree to which the person expects to succeed.

This is expressed as the function:

Motivation = expectancy x value

Source:

Fishbein, M., & Ajzen, I. (1975). “Belief, Attitude, Intention, and Behavior: An Introduction to Theory and Research“. Reading, MA: Addison-Wesley.