The trial that paid people the longest is the one that found nothing. For 18 months it handed out up to $25 a week for progress toward a goal weight. A scheme that ran for just 16 weeks built the biggest lead of the four trials we examined.
The 18-month deal paid by cheque at each weekly weigh-in, scaled to how close each person was to their goal, on top of a group weight-loss programme. The cash didn't shift weight in any way the trial could detect.
The 16-week deal came from a 2008 trial of 57 veterans, led by Volpp. Its paid groups pulled 9 to 10 lb ahead of a comparison group weighed once a month. Three months after the deal ended, that lead was too small to call.
Unlike the weekly cheques, the veterans' deals ran daily. Some staked their own cash, which the researchers matched, and the rest were entered in a lottery drawn every day.
The largest trial, led by Ladapo, included 668 patients, mostly women in low-income neighbourhoods. All of them got the same free year of a commercial weight-loss programme and a monthly one-on-one check-in. Some also got cash, with feedback tied to it, for weight-loss steps or for results. By six months, more than twice as many reached a 5% loss with pay for results as with no cash.
A stake, a draw or a pay slip
The commitment-contract pitch says money works best when you could lose it. The authors of the 1993 trial suspected as much, writing that people's own money may carry more pull, and that a threatened loss may weigh more than a possible gain.
The veterans trial put that to the test. Those who staked their own money lost 14.0 lb over the 16 weeks. Those in the daily lottery, with nothing of their own at risk, lost 13.1. The comparison group lost 3.9.
A chance to win kept pace with the fear of losing.
On days people were on target, the draw gave a 1 in 5 chance at $10 and a 1 in 100 chance at $100. Each scheme was measured against the comparison group rather than against the other, so the two can't be ranked, and the veterans were nearly all men.
The other choice is what the money pays for: doing the work, or the number on the scale. The largest trial tried both on the same programme, and on the scale they came out about level, 4.47 and 4.79 kg lost by six months against 2.21 with no cash. Its authors call the two designs similarly effective.
Where they split was the line. Pay for results got more people over that 5% mark, many of them only just, while pay for the work got more people signed up for the programme and taking part. Pay for the work also paid out more, $440.44 on average against $303.56 from a possible $750: more money, for about the same result on the scale.
What rode along with the money
Here's the catch in all of this. In every trial where money helped, something else came with it, and none of them pulled the two apart. In the one trial where money was the only thing added, it didn't move the scale. Read beside trainers, meal boxes and weight-loss groups, money is the help that only ever worked when it came with company.
The paid veterans phoned in their weight every day and got a message back on their progress and winnings, while the comparison group stepped on the study's scale once a month. In the largest trial, the paid groups got feedback on their cash on top of the programme everyone shared. Its own authors wrote that this may have meant more human contact, which may have affected the results.
If your programme is already working, one pattern is worth knowing. Lined up by what their comparison groups got, three trials that differ in nearly everything suggest money added the most where the programme underneath did the least.
The veterans' comparison group had one dietitian visit and monthly weigh-ins, and the money's lead there was the biggest of the four. The largest trial's comparison group lost about 2 kg by six months, and cash added about as much again. In the 1993 trial, people lost 7.7 kg in the group programme by six months with or without the cash, and up to $25 a week added nothing the trial could detect.
When the payments stop
The harder test comes after the deadline. Three months after the veterans' schemes ended, much of the weight had come back, and what was left of the paid groups' lead was too small to separate from chance. Yet the trial could still tell both paid groups were lighter than when they started, which it couldn't for the comparison group.
In the largest trial, the pay for the work was earned over six months, and both paid groups were still ahead at a year. Only about half the people were weighed by then, though. When the authors tried other assumptions about the missing weights, the gaps in who reached a 5% loss largely shrank.
Nothing in the four trials followed people for years after the money stopped. The camp that says money works and the camp that calls it a bribe that comes straight back are each half right.
Money can also be aimed at keeping weight off. A 1999 trial led by Wing tried it: $25 of each member's deposit went into a jackpot for the teams that kept the largest share of their members at their full loss.
Of those weighed at month 10, 66% of people who joined with friends and got that team package kept their whole loss from months 4 to 10. Among people who came alone to the standard programme, 24% did. Eight people in the contract groups stayed at exactly the same weight over those six months, and nobody in the other groups did.
The average change in weight over those months was no different, though. At a late visit six months after the contracts ended, which about half attended, the groups no longer differed. The money also came bundled with team activities, joining with friends was people's own choice, and the authors called their findings preliminary.
Apps, pots and the size of the stake
If the deal you're weighing is a betting app, a pot you set up with strangers or a family challenge with a prize, it's not one of the deals these trials tested. Two of the four trials did put a pot inside their deals, and the researchers ran both.
The own-money scheme that helped was run by the researchers themselves. People put in up to $3 a day, and the study matched it and added $3 more. It all came back each month only if they were on target, with a message every day saying where they stood.
How big a stake it takes is another blank. No trial in our set compared amounts, and their dollars span 30 years. All four were US trials, 1,093 people in all, every one an adult aged 18 to 70 with overweight or obesity. They measured body weight, with nothing on strength or muscle and body fat only in passing.
Where the money fits
Based on what we examined, money can help people lose weight, but how it's paid decides whether it does. What it bought most clearly was a head start while the money was still coming in.
For anyone staking their own money, the closest test is the veterans' deposit: a strong 16 weeks, then a lead that shrank once the scheme ended. Cash for results added nothing on a group programme that worked either way, and gave a clear boost on a free programme in the largest trial. And the one deal that paid for keeping weight off held a line while it ran.
Everywhere the money helped, someone or something was checking in. So is it the money that keeps people going, or someone expecting them?
One trial asked that about lifting rather than weight loss. Among people who already lift, all on the same 10-week plan, 15 of 17 coached face to face completed 26 or more of the 30 sessions. Of the 47 working from a PDF, 25 did. The trial printed that gap without any test, and it also measured what the training did to their bodies.
The deal that found nothing paid on a ladder, every week for 18 months: $2.50 just for not gaining, $12.50 at halfway to the goal and $25 at the goal itself. The full $25 went only to people who had already reached their goal.
The veterans staking their own cash put in $1.56 a day on average, about half of the $3 allowed. None of the four trials pitted one amount against another, so these are the sizes of the deals that were tested, not a price that works.