there’s a third aspect that also deserves note. Rather than being dripped out in conversations over months, or even years, the intervention compresses everything. It concentrates multiple sources at the same time. All at once rather than over an extended period.
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A few years ago my colleague Raghu Iyengar and I analyzed user growth of a new website.11 Like many new sites, this website didn’t have much money to spend on advertising, so they used existing users to help spread the word. Each user could send out invitations through Facebook, and we analyzed how those invitations influenced whether potential new users joined the site.
Consistent with the value of corroborating evidence, people who got more invitations were more likely to join. Compared to someone who got only one invitation, for example, potential users who got a second invite were almost twice as likely to sign up.
But beyond how many invitations people received, when they received those invitations also mattered. The closer the different invitations were in time, the bigger their collective impact.
To understand why, it helps to go back to the coworker recommending a show. If they tell you how much they love it, and another coworker says something similar the next day, it’s hard not to at least consider checking out the show. It’s a hot topic, lots of people are talking about it, so you infer that the show must be pretty good.
Spread those conversations out a bit more, though, and their effect is muted.
If one coworker says something today, and another mentions the show three weeks from now, it’s less likely to drive action. It’s been a while since you heard about the show, so you’re less likely to infer it’s widely popular. You’ve probably heard about a lot of other shows in the meantime. And if enough time passes, you may not even remember hearing about the show in the first place.
Addiction researchers note that even when multiple friends and family members try to get an addict to change, their efforts are usually spread out. After noticing some erratic behavior, a friend may make an offhand comment. Two months later a different friend may say something else. It’s not until something more serious happens, like an accident or arrest, that a more direct conversation occurs.
But the separation between these expressions weakens their collective impact. If two people say different things at different times, it’s easier to shrug them off as unrelated incidents, or come up with alternate attributions. Forget they happened or discount the last interaction by the time the next one occurs.
Our analysis of user growth found something similar. Each invitation provided some evidence that the website was good or worth joining. Over time, though, it was like some of that proof disappeared or evaporated. Like water steaming off a hot road, the more time that elapsed until the second invitation, the less proof that was left from the first one. After one month, the invitation provided only 20 percent as much impact as it had initially. After two months, it had almost no impact at all. As though people had never even received it.I
But concentration mitigated the decline. Just like hearing the same thing from multiple family members at once encourages action, we found that receiving multiple website invitations within a shorter period catalyzed change.
Take two people, one who got two invitations in quick succession and one who received them a month or two apart. The person who received the two invitations one right after the other was over 50 percent more likely to join the site.
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When trying to change minds, then, not all proof is equal. Concentrating proof boosts its effectiveness.
Trying to increase attention for a new service or important social cause? Make sure that different media hits happen soon after one another so potential supporters hear about it multiple times in a short period.
Indeed, another study we ran found that exposing people to multiple articles in rapid succession about a pressing social issue, such as sexual assault, increased action. It led more people to sign a petition to help sexual assault survivors and increased donations to the cause. Rather than spreading those same articles out, concentrating them in time boosted support.
Trying to change the boss’s mind? After stopping by her office, catalysts encourage colleagues to make a similar suggestion right away. Concentration increases impact.12
When to Concentrate or Spread Out Scarce Resources
Concentration is helpful when trying to change one person’s mind, but it also has implications for larger-scale change. When trying