Contagious Why Things Catch On

New York
magazine ranked it as one of the top twenty food carts in the
city, and people wait up to an hour to get one of Halal’s tasty but inexpensive meals. Go during
certain times of day and the line will stretch all the way down the block.
Now I know what you are thinking. People must wait that long because the food is really good. And
you’re partially right: the food is quite good.
But the same owners operate an almost identical food cart called Halal Guys right across the street.
Same food, same packaging, basically an identical product. But there is no line. In fact, Halal Guys
has never developed the same devout following as its sibling. Why?
Social proof. People assume that the longer the line, the better the food must be.
This herd mentality even affects the type of careers people consider. Every year I ask my second-
year MBA students to do a short exercise. Half the students are asked what they thought they wanted
to do with their life right when they started the MBA program. The other half are asked what they

---

want to do 
now.
Neither group gets to see the question the other was asked and responses are
anonymous.
The results are striking. Before they start the MBA program, students have a broad range of
ambitions. One wanted to reform the health care system, another wanted to build a new travel
website, and a third wanted to get involved in the entertainment industry. Someone wanted to run for
political office and another student thought about becoming an entrepreneur. A handful say they want
to go into investment banking or consulting. Overall, they possess a diverse set of interests, goals, and
careers paths.
The responses from students when asked what they want to do a year into the program are much
more homogeneous and concentrated. More than two-thirds say they want to get into investment
banking or consulting, with a small sprinkling of other careers.
The convergence is remarkable. Sure, people may learn about different opportunities during the
MBA program, but part of this herding is driven by social influence. People aren’t sure what career
to choose, so they look to others. And it snowballs. While less than 20 percent of people might have
been interested in investment banking and consulting going into the program, that number is larger than
any other career. A few people see that 20 percent and switch. A few more see those people switch,
and they follow along. Soon the number is 30 percent. Which makes other people even more likely to
switch. Soon that 20 percent has become much larger. So through social influence this initially small
advantage gets magnified. Social interaction led students who originally preferred different paths to
go in the same direction.
Social influence has a big effect on behavior, but to understand how to use it to help products and
ideas catch on, we need to understand when its effects are strongest. And that brings us to Koreen
Johannessen.
THE POWER OF OBSERVABILITY
Koreen Johannessen started at the University of Arizona as a clinical social worker. Originally, she
was hired by the mental health group to help students deal with problems like depression and drug
abuse. But after years of treating students, Johannessen realized that she was working on the wrong
end of the problem. Sure, she could try to fix the ongoing issues that afflicted students, but it would be
much better to prevent them before they started. So Johannessen moved over to the campus health
group and took over health education, eventually becoming the director of health promotion and
preventive services.
As at most universities in the United States, one of the biggest issues at Arizona was alcohol abuse.
More than three-quarters of American college students under the legal drinking age report drinking
alcohol. But the bigger concern was the