Heard the one about the analyst and mother-in-law?

Apter, a fellow of Newnham College, Cambridge, has spent years trying to understand the tensions that underlie many in-law relationships and in What Do You Want From Me? offers advice on how to manage them successfully.

She acknowledges that many such relationships are harmonious, quoting one mother-in-law as saying: "I don't know how we could have lived all these years without her in our lives". But others regard their in-laws with grim resignation. And where discord is left unchecked, says Apter, it often leads to "the severe deterioration of a marriage". In the UK these tensions often go unacknowledged.

"The jokes deflect the problem without exploring it," says Apter. "We simply don't have the framework in which to think about these problems."

But it appears that we should think about them. The subject has been given more attention in Italy, where the National Statistics Institute found the chances of a marriage lasting went up with every 100 yards that couples put between themselves and their in-laws.

Then there's the Japanese study which shows that women who live with their in-laws are at a greater risk of heart disease, by a factor of three, than women who don't, a phenomenon not seen with men. Why? "The stress of having to negotiate and being of low status, which is closely linked to heart disease," says Apter.

She interviewed 49 couples and their in-laws in the UK and US for her book. She found that while only 15% of mother-in-law/son-in-law relationships had some tension, 60% of mother-in-law/daughter-in-law links were described negatively. But, she warns: "The key fact is that in-law problems are never simple and never involve simply two people."

She noticed that the tensions tend to arise, on the mother's side, from fear of being edged out of her son's life; she may also feel outshone by her son's wife. For her part, the daughter-in-law may feel she is being critically appraised and found wanting.

Why should these tensions arise between a man's wife and his parents but not a woman's husband and her parents? Because, says Apter, daughters are generally better than sons at reassuring their parents their relationship remains special in spite of their marriage.

Some of Apter's stories are so pointed as to be funny. Take Felipe: he has already had one relationship fail, due to his parents' disapproval, when he falls in love with Jess. They decide to marry, but three weeks before the wedding, Felipe still hasn't told his mother Brisha. When they find out, Felipe's parents are "livid" and Jess feels they blame her. On the day, Brisha hands Felipe a present, but Jess's heart sinks when they open it, to find a framed picture of Felipe with his mother, father, two sisters and brother-in-law - but no Jess. Ouch.

Aren't there times when you have to call time on plain old bad behaviour? "Yes, you do have to do that," says Apter, adding with a laugh: "There are cases where the mother-in-law has said to the bride at the couple's own wedding: This is just a family photo - step out of the frame'."

But, she says, while someone like Brisha's motives might be obvious to onlookers, they are usually hard for the woman herself to admit to.

A mother who feels excluded, says Apter, may become intrusive, or may sulk and try to elicit sympathy. In response, her daughter-in-law is apt to become cold. The more the mother-in-law pushes to be included, the more the daughter-in-law will resist. Suddenly, the two women are locked in a power struggle.

But this is typically played out sotto voce, through double-edged compliments and forced smiles. "In-laws trip one another up with underlying messages and projected meanings that can turn an apparently superficial conversation into a minefield," says Apter.

The men in the family "simply do not see the battle being fought before their eyes".

The son may also be loath to stand up for his wife for fear of upsetting his mother.

Apter has observed that the most common response by a man whose wife is demanding "whose side are you on?" is to refuse to discuss the problem; while other responses include insisting criticism of his mother is off-limits or saying "that's just my mother" or "you're imagining things". That's not helpful, says Apter.

"Sometimes the women fight it out among themselves and that makes it worse. The wife will feel his non-involvement as defence of his mother and his mother will think if my son isn't going to stand up to me then I can gain power over my daughter-in-law'."

The son has to realise, she says, that he can stand by his wife and also be a good son. She advises that he should take opportunities both to reassure his mother and to make clear to her that he is loyal to his wife.

She also urges women embarking on marriage to "speak up for yourself" from the start, while making sure the mother-in-law doesn't feel threatened. That way, the question of "whose side are you on?" may never arise.

Case Study :

Family fall-outs? You must be joking LES Dawson's jokes mean nothing to Barbara Brown, who describes her mother-in-law Dorothy as "straightforward, bubbly and kind".

Barbara, 28, a primary school teacher from Musselburgh, has been with Dorothy's son Neil for nine years - they married in July last year.

Barbara immediately hit it off with Dorothy. "There were no tensions," she says. "She's always been so generous.

"She'll get me little presents when she's been away on holiday and thinks about me all the time."

Barbara thinks it helps that they are similar people: "There are no ulterior motives with her. With some people, you feel there are, but she's not like that and I'm not that type of person either."

She says that she and Neil have similar backgrounds, which has made it easier for them to relate to each other's families.

Not only do the two sets of parents see each other socially, but the two families have been on holiday together.

Dorothy, 61, a retired district nurse, says: "Any girl that your son brings home, you're nice to them, but you might think, I don't know about her'. But when Barbara came, we thought, Great'.

"They once came to stay with us between flats for six weeks and she fitted in so well."

She has never feared, as some women do, that she might see less of her son now that he's married.

"I'd rather see my daughter-in-law," she says, laughing. "I try not to be like my mother-in-law was - she thought she was a bit better than anyone else.

"I think we've been awfully lucky with Barbara."