THE billboards advertising Powerball, an American lottery, were not big enough to display the size of the jackpot in the draw that took place on the evening of January 13th: $1.6 billion. That prize will be split among the three winners, who bought their tickets in California, Florida and Tennessee. For several days beforehand, Lotto fever gripped the nation: long queues formed outside shops selling tickets and on the day of the draw sales were ringing up at a rate of $787,000 per minute. Powerball’s website had some advice for its frantic customers: “Swinging a live chicken above your head while wishing for the future numbers does NOT work.”

A more useful bit of counsel would have been that buying a lottery ticket is fun but financially foolish. A punter buying a Powerball ticket has a 1 in 292m chance of winning the jackpot. Buyers are around four times more likely to be killed by an asteroid impact this year. Lotteries are designed to be a bad deal, hoovering up participants’ money in order to plug state budgets and fund good causes.

What’s more, the designers are getting better at their jobs. Victor Matheson, professor of economics at the College of the Holy Cross in Massachusetts, explains that sales are much more sensitive to the size of the jackpots than to the likelihood of winning. After a particularly big prize is won, there is a halo effect, whereby ticket sales remain high even though the jackpot has reverted to the norm. So lottery designers go to great lengths to boost the size of the big prize.

One easy trick is to make the jackpot seem bigger than it is. The sums advertised by Powerball represented the pre-tax value over 29 years of an annuity that winners can opt to receive. If the winners choose a lump sum instead, they get just over 60% of that, on which they would have to pay tax of at least 40%.

Another approach is to boost the jackpot by expanding a lottery’s geographic scope, and thus its potential pool of participants. Powerball and Mega Millions, the two largest American lotteries, have both taken this tack. By forging alliances among state lotteries, they are both now available to residents of 44 of America’s 50 states. Similarly, EuroMillions, a lottery that covers nine European countries, has twice offered a jackpot of €190m ($206m).

Powerball’s record-breaking jackpot stems mainly from a riskier strategy, however. If the chances of winning become so slim that no one guesses the right combination of numbers, the prize rolls over, growing to a vast sum. Both Powerball and the British national lottery changed their rules to this effect in October, by increasing the number of balls in the draw. In Britain the change slashed the chance of a winning guess from 1 in 14m to 1 in 45m. In America it fell from 1 in 175m to 1 in 292m.