Saying "I doubt this startup will succeed" is reasonable, but that's not the same as saying that a startup idea is stupid. More importantly, startup returns ar

Saying "I doubt this startup will succeed" is reasonable, but that's not the same as saying that a startup idea is stupid. More importantly, startup returns aren't like roulette returns. Say we hold a friendly roulette game where the house doesn't take a cut (to simplify). If you bet $1 on every number you win nothing. On the other hand, if you're an angel investor and you bet on 100 startups, you win as long as a couple of startups hit big. Being wrong 95% of the time can really pay off. Of course, what is rational depends on your goals. Some people get nothing even if they "believed" in an early startup. For example, it's possible OP wasn't a potential investor and as such wouldn't have gained much by holding a different belief about Pinterest anyway. Also, some stock analysts and tech journalists get away with making bad predictions, as they manage to point out only their good ones.