What? You Don’t Love Your Mother-In-Law?

This is how it begins: Your guy pops the question and tells you that his family is just going to love you, especially his mom. You're the daughter she never had. You assume that means she's going to be supersupportive of all your choices, will offer help when you ask for it, but otherwise, stay out of your life and marriage. How perfect.

Meanwhile, his mom has a fantasy of her own. She assumes that since you're so crazy about her son, you see her as an authority on marriage and children—and her son. Of course you'll want lots of advice from her because you want to be just like her. She can't wait to start "helping."

Call it the clash of the fantasy lives. The result: 60 percent of women use words likes "strained," "infuriating" and "simply awful" to describe their mother-in-law/daughter-in-law relationship.

If you listen to mother-in-law jokes—and there's a lot of them—you'd think that the main combatants were sons-in-law and mothers-in-law. But based on years of research, Cambridge University psychologist Terri Apter says it's now clear that this is primarily a woman-to-woman problem. In her new book, What Do You Want From Me? Learning to Get Along With In-Laws (Norton), Apter says most in-law problems can be traced back to unspoken but conflicting expectations and assumptions.

For instance, a lot of daughters-in-law assume that no matter how modern their mothers-in-law are, they are judging them based on the standards of traditional housewives: the ability to keep a clean house, be a good cook, and raise respectful children. At the same time, mothers-in-law often interpret the decisions of their daughters-in-law to do things differently as a rejection of their own choices. Daughters-in-law assume that as a fellow woman, their mother-in-law will be their ally when they disagree with their husband. But guess what? Her role as his mom trumps the call to sisterhood.

It's the disappointment felt by both women that "gives these relationships their distinctive negative charge," Apter says. Add to that a mother's conflicted feelings of pride and loss as a son marries; a wife's insecurity that she's adequately balancing work and home responsibilities, and the tendency of most women to be more sensitive to slights and criticisms than men, and you have the formula for years of trouble. In some respects, Apter says, the ensuing jockeying for position has a lot of similarities to the games "mean girls" play in middle-school hallways. As Apter sums up in her book: "Each is the primary woman in her primary family. As each tries to establish or protect her status, each feels threatened by the other."

And what does the son/husband do to ease the tension? Typically, not much, says Apter. Men tend to ignore, or are oblivious to, the little digs and disses that pass between their mothers and wives, and don't want to get involved. "Daughters are better at reassuring their fathers that they are still their darling little daughter and will sustain that role, even as their lives change and they draw new boundaries. Sons are not as good at reassuring their mothers that they will continue to have a role in their lives or confronting her and saying new boundaries are needed. If they fail to do that, the negative conflict is played out between the women."

These tensions do more than cause friction within families. They can put even the best marriages at risk. An Italian study, for instance, done by the National Statistics Institute, found that the odds that a marriage will last increase with every hundred yards that couples put between themselves and their in-laws. Italian courts found this evidence so compelling that they have ruled that a wife has the right to a legal separation if her husband is not effective in preventing his mother from "invading" their home, Apter says.

In-law troubles can also increase stress, and even impact health. The most extreme cases may be seen in cultures where newly married couples are expected to move in with the husband's family and the bride is expected to be subservient to her in-laws. A Japanese study published in 2008 in the journal Heart found that women living in multigenerational households (grandparents, adult children, and young children) were two to three times more likely to experience coronary heart disease than women living with just a spouse. Are you surprised to learn that the men living in those multigenerational homes experienced no increased risk?

Although she's a psychologist specializing in family relations, Apter admits that she has fallen into some common in-law traps herself. An American who married a Brit, Apter said she assumed that she and her husband "were forming an independent couple" and that her husband's parents would not have a big impact on their lives. She soon found out how wrong she was. And years later, after raising her own children and seeing them marry, she had to make the "very difficult" transition to being a mother-in-law herself. "I now am very aware how important the relationship with your children remains throughout your life, no matter how old or independent your child is," she says. "I can also see that as much as the newcomers enrich our family and are crucial to my children's well-being and the continuity of our family, I still get concerned about what's right for my children, and I worry about whether their spouses put them first."

Maybe she's not the world's best mother-in-law yet, but Apter's working on it. In the meantime, here's her advice on avoiding the most common pitfalls: