Behavioural science is the study of human actions. It attempts to deconstruct, understand and predict why people behave in the irrational ways that they do. It consolidates knowledge and evidence from diverse disciplines including (but not limited to) behavioural economics, psychology, neuroscience, sociology and anthropology.
Behavioural science challenges the more traditional notion that people always behave in ways that tend to maximise their self-interest. Instead, it suggests that behaviours are often a consequence of the physical and social environments in which these actions and their associated decisions are made. Much of this decision-making happens at a subconscious level.
But behavioural science is not a one-step solution for all kinds of problems.