Save just finished making a great new website. You build a contagious message so that people will share. But you only have enough money to diffuse it to a certain set of your target demographic. Should you spend all your money targeting one group? Should you spread it out? Within that group, who should you target, and why? Should you pick people that are popular? Maybe that have lots of friends, or people that have the right position in the social network and might connect you to other individuals in different networks. One way to think about this is a sprinkler or waterfall strategy. A waterfall strategy, when you concentrate all your marketing efforts in one place or one region. If in the United States, for example, you might pick a given city to start in, concentrate all your resources there, and then hope to spill over to the nearby cities moving on. A sprinkler strategy is when you spread out your resources, different areas, rather in concentrating 100 percent in one city, you might spend 10 percent in each of 10 different cities. Which of those things might be more effective and why? We can think about reasons for both, are the benefit of the sprinkler strategies you're spreading it out. You're starting many seeds in many different communities. That could be good because it could get it to catch on faster in different networks. But a waterfall strategy could also be good if people need multiple doses of influence before they adopt something. One way to think about whether to use a sprinkler strategy or a waterfall one is to understand whether that product that you're hoping to catch on is a simple or complex contagion. A simple contagion is something that you only need one dose of influence before you're willing to adopt. Take for example, a newspaper article that someone might send you online. You don't need to hear about that article from five or six people before you're willing to open the link, one person is enough for you to check it out. A movie takes a couple more doses of influence. You might not just go see a movie because one person said something about it. But if a few people say something about it, you'll check it out, and on the much more complex end of the spectrum, say someone suggested a new open-heart procedure. You wouldn't just need one person to tell you about it before you try it. You want to do a lot of research before you're willing to try that risky procedure. So complex contagions require more doses of influence before you're willing to adopt, whereas simple contagions take fewer. Take for example, a pickled pickle. You only need one dose of influence to try a brand of pickles. But if someone said try pickle broccoli, you might need to hear about it a few people before you're willing to try it out. Again, simple contagious are things that require only one dose of influence, whereas complex require multiple doses before people are willing to try it. Complex things tend to be more costly, whether in terms of time, effort, or energy. The more money you have to spend doing something, the more you want to get to hear about it a few times before you're willing to do it. Well, simple versus complex contagions have an important implication for whether you should use a waterfall or a sprinkler strategy. When you have a complex contagion, you need to use more of a waterfall strategy. When people need multiple doses of influence before they're willing to adopt, they're going to need to hear about that product or service from multiple people before they feel comfortable. Which means you need to make sure that multiple of their peers have heard about it to get them to change their mind. That suggests you need to concentrate your resources in one geographic, demographic, or social network region online, making sure that into people on an individual area, a subset of the network have heard about it multiple times. If you've got a simple contagion, if people only need one dose before they're willing to do it, well then you can spread your resources out. People don't need to hear about it multiple times, and, in fact, multiple times we'll waste some of your marketing resources. It's much better to spread them out, so that each person hears about it once, they hear about it from one other person that causes them to adopt it and then increases the likelihood of catching on more quickly in the broader community. The more doses required for product adoption, the more you want to concentrate your resources in one area, one geography, one social group, or one set of people that have similar tastes.