The difference between normal jealousy and abnormal jealousy is this: Truly jealous people experience it with practically every person they date. They could date Mother Theresa and still be convinced she was squeezing in a bit of hanky-panky on her way home from the orphanage. In fact, most jealous people know they have a problem. Once they've calmed down, most also know they've been illogical. The trouble is, when you're in the middle of a jealous rage, you lose all sense of perspective. Jealousy is an incredibly powerful emotion. You can't control it because it's being fed by one continuous thought: Is my partner being unfaithful to me?
Jealous people look at the world through distorted lenses. They see danger where there really isn't any. It also has a lot to do with self-esteem. If you're happy with yourself, you're much more likely to think Why would my partner want anyone else when they've got me? A healthy ego is great protection against jealousy.
Extreme jealousy is an ugly emotion. A doctor friend of mine tells the story of a woman in her thirties with terminal cancer who had to spend a lot of time in bed. She bought some pretty pajamas so she could look good for her husband, but instead of complimenting her, he flew into a jealous rage and accused her of trying to look attractive for the hospital staff. A psychiatrist told me about a patient who wired up his entire house with microphones so he could tape the supposed sex sessions his wife had while he was at work. They weren't rich and he spent their life savings doing it. All he recorded was her doing the dishes and gossiping on the phone to girlfriends. (So much for that vacation they'd been saving up for.) There's another quite bizarre case of a man who was so jealous, he was convinced his wife was being unfaithful during the few minutes he left the bed during the night to use the bathroom.
The couple where one or both partners are jealous is easy to spot: They're the ones who stay home. The only stress-free environment for that kind of a couple is on the couch with a DVD and a bottle of wine because any public place—pubs, restaurants, even shopping—has potential rivals (i.e. people of the opposite sex). But some people can't even bear to have their partners watching attractive people on television and in movies. Even the news becomes a problem if the newsreader happens to be attractive.