Most punters are unaware that betting firms, whose adverts promise glittering riches, are at pains to shut out winners. If pressed, the firms stress that very few players are restricted. A report by Britain’s Gambling Commission found that 4.3% of accounts active in the past year have “stake factors”—the proportion of the maximum bet offered to a new customer—below 100%. American books give a longer leash: only 0.64% of accounts in Massachusetts are limited, for instance. But restrictions are rare only because most customers are “square”—that is to say, losers.

No one knows how many people make a living betting. However, they range from stealthy lone wolves to “syndicates” as big as a hedge fund. Contrary to popular perception, advantageous wagers are everywhere if you know how to look for them. The profit margins sportsbooks build into their odds are as low as 4.5%, and they accept bets on far more events than their algorithms can price accurately. So their business model works best if they strive to identify and restrict sharps. “Betting is easy,” says Antonino de Rosa, whose syndicate has 17 employees and wagers an average of $12m a week. “Betting a lot of money is hard.”

A good “player-profiling” strategy can boost a betting firm’s margins by 10-20%. It is not simply a case of shutting out people who win large sums. Such folks may be lucky rather than skilled. If so, bookies want to give them a chance to lose it all again.

At the same time, they want to hook big losers, or “whales”. When betting firms spot punters who appear deep-pocketed and reckless, they raise their limits and offer them vip treatment to keep them playing. Such inducements are especially lavish in America, where rules to protect problem gamblers are comparatively lax. However, some “whales” are sharps in disguise.

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The profiling process starts before you place a bet. Are you using a phone, like most punters? Good. Or a computer, which makes it easier to compare odds? Not so good. Did you deposit by debit card, or via the e-wallets preferred by syndicates? Are you a woman? That is suspicious. Far fewer women bet than men, and many sharps get women to place bets for them.

The first wager you place speaks volumes. Normal punters bet on the most popular spectacles, such as English Premier League football or America’s National Football League (nfl), starting around half an hour before kickoff. They generally bet on who will win, what the scoring margin will be and which statistical milestone a star player will achieve, paying little attention to the odds. Square players love to combine multiple bets into a “parlay” or “accumulator”, which delivers a big payout only if all of them win.

Sharps have the opposite tendencies. They target less popular leagues and bet as soon as odds are published, when they are most likely to be mispriced. They shop around. They like obscure “derivative” markets, such as how many points will be scored in the third quarter, and bets on lesser-known players to perform poorly. They rarely use parlays. They make big deposits, and seldom withdraw winnings. “By the time a customer places his first bet, [sportsbooks] are 80-90% certain they know the lifetime value of the account,” says Ed Birkin of H2, a gambling consultancy.