Yesterday's news of a Lancing couple's death in an apparent suicide pact was tragic, but theirs was by no means an isolated case.

On Monday night disabled Edmund Tuvey and his wife Margaret Morton were found dead at their home in Monks Avenue, Lancing, in an apparent suicide pact.

Neighbours said they were devoted to each other, although it is still unclear as to why they would want to take their own lives.

They appeared to have swallowed tablets and died together in the bedroom of their bungalow.

It was the latest in a string of double tragedies in Sussex.

In December, friends Elizabeth Gold, 16, and Jennifer Prinn, 17, gassed themselves in a car in a friend's garage in Crawley.

They had grown up together in South Africa and may have made their decision because they were homesick, an inquest heard.

Many more couples who have made suicide pacts have been elderly.

In January Kenneth and Alice Hamilton, both 77, were found dead at their home in Cudlow Garden, Rustington.

Last November, Ida Dormer, 90, and husband Edwin, 92, tried to kill themselves by taking an overdose of paracetamol at their home in Littlehampton.

Mrs Dormer died in hospital but Mr Dormer survived to tell an inquest they could not go on after his wife kept falling over and injuring herself.

According to researchers, suicide pacts are rare. Detective Inspector Paul Williams, of Worthing police, said it was the elderly who were most likely to make them.

Mr Williams, who has a degree in psychology, said: "In this area, there have been cases of elderly couples choosing to end their lives together. Perhaps one of them is terminally ill and they cannot bear to live without each other.

"It is quite unusual for younger people to do this. It's more likely to be people towards the end of their lives."

Mr Williams declined to comment on the latest case, but said from his experience: "It is a little more complex than just having a problem. There is often a whole range of things happening in a person's life and something may happen which is the straw that breaks the camel's back."

He has investigated a number of double suicides in his career and said not all of them were planned.

He said: "Sometimes people fall into a trough in which they can't see things getting better. For ages they cope and get on but one particular day or for a number of days it just becomes too much."

But why do people choose to die together? Why doesn't one talk the other out of it?

Research is scant and it appears many people who attempt to die together in some form of pact succeed. There are no figures available in Britain.

Dr Geoff Searle, a spokesman for the Royal College of Psychiatrists, said: "Often, when an individual attempts to kill themselves they will fail because of the chaotic state of mind.

"But when there are two people involved they seem to succeed, maybe because there is one of the two who is more organised and not in such a chaotic state of mind."

American research from people who have survived suicide pacts suggests that suicidal people often encourage each other in their suicidal behaviour.

They discuss their thoughts and feelings openly with each other, which may include violent and suicidal thoughts, until eventually suicide becomes a little less frightening.

The American research suggests that at one time, people who died in suicide pacts may have attempted to encourage each other to live but eventually lost hope.

Dr Searle said: "Suicide pacts are very unusual. You may come across one in 15 years. There is the Romeo and Juliet issue of star-crossed lovers taking their own lives but it really depends on the nature of the relationship.

"There is often a leader and a follower in a relationship. If the stronger half of a partnership has problems then the other half may not be able to bear being alone and may feel they are unable to cope."

Dr Andy Field, a lecturer in psychology at the University of Sussex, said people with poor coping mechanisms might be susceptible to suicide.

He said: "There's all manner of vulnerability factors as to why people kill themselves. People's inability to solve problems and their sense of hopelessness may be a factor.

"Some people can't see how to solve problems in the way most of us think and cannot work through short-term problems. Sometimes, a specific critical incident occurs, which will break the camel's back and people say, 'Right I've had enough'."

An inquest into the Mortons' death will be held, but even then it may not be clear why the couple decided to end their lives and leave their loved ones behind.