Ever had the feeling that you're acting out of character? Maybe you have lost your temper or become a very loud drunk, done something that you think is not really you'. Reading Rita Carter's Multiplicity should shake up that belief, especially if you think you' are one single entity.

She argues that you are not out of character', but merely expressing one of your personalities; one that you barely know because it so seldom appears.

I'm not sure which Rita I'll meet when first arriving at her fabulous flat in the depths of rural North Oxfordshire. I have already met the award-winning writer in print. Her book is well researched, brilliantly written and totally fascinating, and it has already impacted on my own life.

Some books you read in this job and they actually make you stop and look at your own life. This is one of those. So I'm looking forward to meeting this author who has made a career out of different areas of the brain.

Rita lives on the ground floor of a stately home and I see another personality in the gold lamé curtains, the deep red walls and eclectic furnishings. This is a woman with her own magnificent bohemian style. But there is also another Rita on display.

On white paper on walls around are various messages. The one on the mirror says "Just Get On with It!!!", underlined in red. It works brilliantly as a visual image of someone's mind and is a pretty excellent ice-breaker. It's good to know that successful authors suffer from procrastination too.

Rita's background is in journalism.

"I actually did that very old-fashioned thing of working for the local newspaper," she said. It was the Wembley News and Observer and they trained her for four years. "By the time people started coming out of universities with degrees, I'd got five or six years of reporting behind me, so I got a good head start," she said. She then worked for every type of media, before co-presenting the 6.30pm news programme for the now defunct broadcaster Thames TV. So why did she move into medical journalism?

"I just had an absolute fascination for science," she said. "So when I got the chance to write about it, I just set myself up. I started writing health things and was a health page editor for She magazine." She wrote a lot about women's issues, but what interested her more was what she terms the harder stuff, the heart and then the brain.

"Once I hit the brain, well, you can stay there for any number of lifetimes," she said.

It was during the early nineties when neuroscience was just starting to expand.

"It's the only time in one lifetime that I feel a discipline has been born and grown in the way it has." She started looking for a book that would pull together all the information that was appearing about the brain. After a while, she pitched an idea about writing one to her agent and Mapping the Mind was the result. This was followed by Exploring Consciousness, Mapping the Memory and now Multiplicity; each book growing out of the one before.

"We do not, despite our strong intuition, have a single unitary or continuous personality," she argues. "The only thing that gives us this idea that we are one person is that we remember what we were like yesterday and we can imagine what we were like yesterday. But most of us change much more."

This is not multiple personality disorder (MPD), which Rita discusses in depth. Unlike MPD, you will know that you are switching. Some people change personality to help them get through the day.

"One very classic way of looking at it is that one person has their home and work life," she said. "Another one is people, particularly those from other cultures, who have a set of friends from their old culture and another from their adopted culture."

Where it gets unhealthy is when you cannot stop behaving in a way detrimental to your health and relationships. Multiplicity is not just a scientific explanation about your different personalities, however. Half the book is a practical self-help guide, suggesting how to become aware of and change personalities if they are having a negative impact on your day-to-day world.

Rita argues that we have developed varying personas because of our dizzy pace of life.

"If you look back only 50 years ago, most people were brought up in a very consistent and continuous environment. The more changeable one's life is, the more likely you are to develop different personalities."

This is a good thing, she argues, as adapting helps us survive better. Good it may be, but worrying for certain women. You will no longer be able to pin on PMT the frothing, yelling, monthly witch that emerges; that really is just one of you!