The ties between brother and sister are unique; parents usually die before siblings, marriages are frequently sundered by divorce, children move away, but brothers and sisters remain bound by blood and the indelible shared experiences of childhood. In her new book, Mixed Feelings: Love, Hate, Rivalry and Reconciliation Among Brothers and Sisters, writer Francine Klagsbrun argues that early encounters between siblings—spats, intimate play, feelings of anger and jealousy—have an effect on our adult lives as profound as the parent-child relationship. The author of Married People and other books on family issues, Klagsbrun interviewed and surveyed more than 400 people, aged 19 to 71. A New York native who lives in Manhattan with her husband, Samuel, a psychiatrist (they have a daughter, Sarah, 22), she spoke with reporter Jane Sugden about a topic so powerful for her subjects, says Klagsbrun, that "boxes of tissues walked out of my office. It opened up floodgates of emotion."

Did childhood rivalries with your brother inspire the book?

Yes. I remember a Sunday when I was 6 and my brother Robert was 10. We were eating" ice cream during a hot ear ride, and I was determined for mine to last longer, so I let it melt. Finally, he said he'd finished, and I drank mine in niter triumph, thinking for once I had bested him. But then he called out from the backseat, "Ha, ha! I still have mine." Again he had outdone me. Decades later, my husband pointed out how I compete with him as I did with Robert. If Robert was so significant in my life, this had to be true for others as well.

Do you believe psychologists home downplayed the importance of siblings on our psyches?

It all goes back to Sigmund Freud, who was an eldest, favored child and so emphasized his relationship with his parents. That relationship is crucial, but one can be just as permanently influenced, in as important ways, by a sibling.

How do siblings shape our adult lives?

As children, siblings are rivals for their parents' love and attention; they also fight for power over one another. These patterns established in childhood then gel replayed in sometimes destructive ways. In my book, I describe a woman who twice married and divorced dynamic, irresponsible men like her younger brother. Parents may treat children the way they treated their siblings. For instance, you might be more irritated In a child who reminds you of a sibling you really didn't get along with.

What about outside the family?

If you came from a very competitive family, you would likely act the same way with coworkers and feel inordinately jealous if someone gets a deserved pat on the head. If you spent a childhood competing with your sister for your mother's attention, you might always try to be the closest to the most popular person in your circle of friends.

Do birth order and gender matter?

Being the oldest or youngest child certainly influences your identity, but it goes beyond just that. The oldest may be someone who is always bossy with people and has difficulty making friends; the youngest may even become the most confident because they've had all this love and attention. With gender, sisters often become the closest of all siblings, whereas brothers tend to be more competitive with each other.

How can we break these patterns?

First, be aware that something is wrong in your present life and then look into your relationship with your siblings and then back into your childhood. The next step is to speak to your sibling about that history and be specific about what you want to change in your relationship. Then try to move on to reshaping how you behave with others.

Why is it so important to maintain sibling relationships?

More than ever before, we live in an alienating world where divorce is rampant and there are few people, including spouses, we can count on. Baby boomers are having smaller families than couples did in the past, which means there's a good chance that as we get older we'll have more siblings than we have children. So we need brothers and sisters to support us emotionally. For instance, elderly people feel a sense of comfort and security when they know that they have a sibling that they can turn to. It helps ward off depression.

If society is moving toward fewer children and fewer siblings, what of the benefits that come with large families?

We definitely lose out by not having more brothers and sisters. There's less family warmth and camaraderie, fewer lessons in accepting differences and compromising, fewer people to lean on. So it's especially important to reap all the rewards you can from bonds you do have. As one woman said to me, "There's a kind of laughing you do with a brother or sister that you can't with anyone else."