rational choice
When you stand at a coffee counter and pick the medium over the large because it is enough and cheaper, you are weighing what you get against what it costs and choosing the option that serves your goals best. Economists assume that, much of the time, people behave this way: they have aims, they consider the costs and benefits of their options, and they pick the one that gives them the most of what they want. That working assumption is called rational choice.
Rational choice does not mean cold, selfish, or always correct. It means consistent and purposeful: a person has preferences, faces constraints (a budget, time), and chooses the option that, given their beliefs, best advances their own goals — whatever those goals are, including kindness or faith. In this framework someone who donates to charity is still 'rational' if giving is what they value most for that dollar. Economists build this assumption into their models because it makes behaviour predictable enough to analyse — for example, predicting that when a good gets cheaper, people buy more of it.
Rational choice is a useful simplification, not a literal description of every brain. Real people use rules of thumb, are swayed by how choices are framed, procrastinate, and make systematic mistakes — the whole field of behavioural economics studies these departures, sometimes calling people 'boundedly rational'. The honest position is that rational choice is a good first approximation that explains a great deal, while being wrong often enough that economists keep a humbler, evidence-based model in their other pocket.
When petrol prices jump, many people start carpooling, combining errands, or buying smaller cars. No one ordered them to; given their goals and a higher cost, the rational response is to use less. Predictions like this rely on rational choice.
Higher costs nudge purposeful people to economise — that's rational choice.
Rational doesn't mean selfish or always right; it means purposeful and consistent. Real people often deviate — that gap is what behavioural economics studies.