This is the adjective for a person, an action, or a reason that is driven by what the person gains. 'A self-interested decision' is one made because it helps the decider. It is quieter and more formal than 'selfish'. If you call someone selfish, you are being rude to their face and talking about who they are. If you call their decision self-interested, you sound like an analyst: you are simply naming the motive, and it is a criticism you can make in a meeting or a newspaper without shouting. That coolness is exactly why writers reach for it. It appears most in serious writing — politics, business, philosophy — and it is often used about groups and institutions, not just people: self-interested politicians, self-interested lobbying, self-interested behaviour. It usually leans negative, because pointing out someone's motive is usually a way of doubting them. But in economics it is used flatly, with no blame at all: a model may simply assume 'rational, self-interested actors', meaning people who choose what is best for themselves. Note the shape too — it sits before a noun very comfortably, and after 'be' as well: 'his motives were purely self-interested'.