"At least, these are the results coming in from long-term studies of social networks - the networks of friends and families, neighbors and colleagues that we all belong to. Such studies have found that one person's change in behavior ripples through his or her friends, family and acquaintances. If one of your friends becomes happy, for example, you're more likely to become happy too. If you're great friends with someone who becomes obese, you're much more likely to become obese as well. " In this excerpt from the NYT's the author is using which type of development?