A smile can reveal different things about a person. For example, researchers now say that the biggest grinners in old photographs were the least likely to get a divorce later in life. Overall health can be seen in a smile. It can show fertility and happy teenagers earn 10% more income than the average 29-year-old. It could also be a smile of power. A study found powerful people can smile whenever they please, but less people powerful people smiled when they were obligated to. It also shows weakness when it comes to professional fighting. So if you're negotiating and smiling, you may not win that fight.