Behavioural ethics is the study of why people make the ethical and unethical decisions that they do. Its teachings arise from research in behavioural psychology, cognitive science, and evolutionary biology.
Behavioural ethics is different from traditional philosophy. Instead of focusing on how people ought to behave, behavioural ethics studies why people act as they do. Arguably, it is more useful to understand our own motivations than to understand the philosophy of Aristotle.
Research in behavioural ethics finds that people are far from completely rational. Most ethical choices are made intuitively, by feeling, not after carefully analyzing a situation. Usually, people who make unethical decisions are unconsciously influenced by internal biases, like the self-serving bias, outside pressures, like the pressure to conform, and by situational factors that they do not even notice.
So, behavioural ethics seeks to understand why even people with the best intentions can make poor ethical choices.