Anyone married for more than a few years can tell you that you need to work on it daily. We all want a happy marriage, but it’s easy to fall into a rut as the years fly by. Respect, honesty, and communication are some of the keys to making it work. But there may be a shortcut to ensure a couple’s happiness. European researchers have studied hundreds of couples to learn the keys to a successful union. And it comes down to one thing: PDA, which refers to public displays of affection. For the most part, the type you choose to display doesn’t matter. Kissing, hugging, or even cuddling will make a difference…