Behaviourism is one of the four lenses of game making, and is the school of thought that prefers experient, rule-driven designs. It is inspired by behavioural and motivational psychology, and considers all games as challenge, anticipation and reward engines.
Behaviourists model their games on psychological hooks that open loops, draw engagement and encourage emotional attachment to outcomes. They use repetitive actions to complete those loops and deliver rewards. The anticipation of a loop’s end, and the reward, has a powerful effect on the human mind and can engender feelings of optimism.
Behaviourists measure everything that they possibly can about their players, test small changes and then measure their outcome. However this means that behaviourists tend to be wary of emergence. Emergence is generally hard to directly measure, and to the behaviourist anything that cannot be measured cannot be reliably improved.
So behaviourists tend to be the most creatively conservative of all lenses. They consider it better to copy a successful game and improve upon it, or adapt another game (perhaps with a different theme) rather than create from scratch. This sort of lean and predictable approach is why behaviourists are the darlings of the investment scene, but also why their games tend to be lower value on a per-user basis than all other kinds.