Mathematician: Gambling (at casinos, in lotteries, and in most other instances) is expected value negative, even when you play optimally. you will lose money, on average). It also implies (via the Central Limit Theorem) that when you play many times, there will be a greater than 50% chance that you lose money overall. Betting against friends can also be expected value positive if you can be sure you’re really a better gambler than they are. All of this being said, if you do decide to gamble, you’d better study up on your probability!