It's normal to feel jealous in some situations, but what if it's all of them? When does it cross the line? I'll explain when jealousy is appropriate, when jealousy is inappropriate and when it's just plain extreme

Where it comes from

Trust is quite possibly the most essential ingredient in a good relationship. You can have matching libidos, you can have the same goals, dreams and desires. You can both enjoy wearing space suits during sex! But without trust, it all means nothing.

For some people, trust comes easily. They work on the assumption that someone will be faithful until they're proven wrong. If this is you, count yourself as one of the lucky ones.

You might get a nasty surprise now and again, but at least you're giving your relationship the best chance to succeed. Giving someone your trust means putting your heart in their hands for safe keeping. It's a precious present and it takes guts to do it.

Then there are the rest of us: the battle-scarred. If you've been hurt before, or if you grew up with parents who cheated, trusting your partner can seem as foolish as standing in the middle of a four-lane motorway and expecting not to get hit by a car. Others hover somewhere in the middle. But how do you determine what's a 'healthy' level of jealousy and what's getting way out of control?

The good

Everyone gets jealous occasionally. If you love your partner and you feel someone is threatening to take them from you by flirting, it's normal to feel a possessive pang of jealousy, one that's usually sorted out in a second with a bit of reassurance.

Other times it's just that you've chosen the wrong person. Match a 'normal' person up with someone who presses all the wrong buttons and just watch all that 'normalcy' go right out the window as he or she become outrageously, irrationally jealous.

Sometimes jealousy is warranted. If your partner has a history of playing around or if he's done something to justify your mistrust - cheated on you or has a history of cheating - you're wise to keep your eyes open.

The bad

The difference between normal jealousy and abnormal jealousy is this that 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.

The ugly

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 pyjamas 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 holiday they'd been saving up for!

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 sofa 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.

The saddest thing of all is that jealousy not only won't stop people from being unfaithful, it makes it more likely they will be. The second biggest fear of a jealous person is that her partner will leave her, but if she continues to make his life hell, he probably will. Don't let this happen to you.