Our five-and-a-half-year-old daughter, Jessie, is curled in my lap—as much as any five-and-a-half-year-old child can curl in her mother's lap. Her long, gangly legs don't quite fit anymore. She's got one thumb in her mouth, the other hand pulled tight to her chest, and her face is resting against my breastbone—the fetal position, kindergarten-style.

Across the room, Annie, our eighteen-month-old, sees Jessie in her favorite spot and blows a gasket. Her brow furrows and she waddles toward us like a mad duck, her round belly clearing her path as she steps on strewn toys and bangs into the table, then a few chairs, en route. She's making a noise I can only describe as keening, and I'm reminded once again of the wild intensity—and sheer absurdity—of this thing called sibling rivalry. A keening, waddling, pissed-off duck is coming at me as a delicate, too-tall flower lies limp and sad in my arms. This can't end well.

Annie reaches us, and with one meaty-armed shove—surprising in force for someone who weighs as much as a holiday turkey—pushes Jessie, who falls from my lap in a long, drawn-out tumble, like an origami swan coming unfolded. Writhing on the floor, Jessie yells, "I hate her!"

Annie smiles, climbs into my lap, tilts her sweet face up to me, and chirps, "Book?"

William, our nine-year-old son, saunters by on his way to the fridge for a snack, notices Jessie crying, and says off-handedly, "She just wants you—like we all do."

And, with that, our big, bookish boy sums up the whole mess. A natural historian peering in through our kitchen window might call the episode a clear case of the survival of the fittest. An economist might deem ours an obvious example of competition for scarce resources. And a writer might wax poetic on the positive outcome of all that sisterly competition, as Simone de Beauvoir once did: "She helped me to assert myself … I believe I should count the fact of having had a sister, younger than myself but close to me in age, as one of my pieces of good luck."

Brother-brother, sister-sister, and brother-sister relationships have been mucking up the works ever since Cain fell out with Abel. But, as a field of study, sibling relationships are relatively new. While Freud probed parent-child and husband-wife bonds, the study of siblings has long existed in a kind of academic backwater. But, with a rise in new ways of looking at these age-old relationships in fields as various as evolutionary psychology, sociology, biology, anthropology, and even zoology, siblings have taken their rightful spot amid the forefront of the study of families. What has emerged can be as thorny as it is fascinating. But at their best, such studies shed new light on these fundamental relationships, which, perhaps more than any other, shape who we ultimately become.

This fall, a new book was published that condenses much of this research in one volume. In The Sibling Effect: What the Bonds Among Brothers and Sisters Reveal about Us, published in September, Jeffrey Kluger, a Time magazine writer, gathers the latest sibling research and intersperses what he learns with his own memories of growing up with three brothers, and later, a half-sister and half-brother. The result is a rich, thought-provoking mix of social science and memoir.

Looking back on his childhood with three brothers, Kluger often waxes nostalgic. "The four of us, we came to know at a very deep level, were a unit," he writes, "a loud, messy, brawling, loyal, loving, lasting unit. We felt much, much stronger that way than we did as individuals. And whenever the need arose, we knew we'd be able to call on that strength. Even now, several decades on, we still can." But the point of his book goes much deeper: Those who don't feel such warmth toward their siblings have also been shaped in fundamental and inescapable ways by them, whether they like it or not. We may believe that once we leave home and the daily company of our brothers, sisters, step-sibs, and halfs that we're free of their influence. But the new research suggests that that is far from the case.

Eighty percent of Americans grow up with at least one brother or sister, and our bonds with our siblings are often the longest lasting and the most intense of our lives. In contrast to parents, whose relationship with us is more authoritarian in nature, our siblings swim alongside us in the family pool. We take baths with them, share bunk beds, kick each other under the dining room table, and wrestle across the back seats of station wagons through the crucial years of our young lives. From such intense, abiding bonds grows a family tree so tangled, so beautiful, and sometimes so bruised that those of us with siblings see traces of them, like a fine dust, on everything we do—even, or perhaps especially, when we become parents to siblings ourselves. In my case, I am particularly attuned to how my kids play—and fight—with one another, because I lost a brother when I was nineteen, and it wasn't until long after he was gone that I realized how much his life had influenced my own. Will it take a lifetime—and a tragedy—for my own children to reach similar conclusions?

Turning to Kluger's book not just as a sister but also as a mother, I was sometimes left with more questions than answers. Like: What power, if any, do I have in shaping my children's relationships with one another? Should my husband and I try to promote loving bonds among our three kids, or is the degree and type of connection they share pre-ordained by birth order, age spacing, and temperament? Does the way they act toward one another now affect how they'll interact later in life? And, most importantly for me these days, when all hell breaks loose, should I step in and referee, or can I let them pummel one another in the other room as I drink my coffee, serene in the knowledge that their relationships are their own and have nothing to do with me? For answers, I dug more deeply into the history of siblings, and the research itself.

For decades, sibling research lagged behind other kinds of psychological and sociological family study. In large part, that's because it is so hard to do. Unlike probing a simple two-person relationship—say, a mother and child—the study of siblings is rife with variables. Researchers have to take into account such elements as age, background, gender, and overall family size and structure. And, not only is each family different from one another, each one changes, itself, in multiple ways over time. Added to that, the increase in divorce and remarriage in modern times have blurred family boundaries. So now, for the purposes of research, who do we call a sibling? For those of us in blended families, how do we explain which branch we occupy on the family tree? In my case, for instance, my parents divorced and my father remarried, so I grew up with two full brothers who were nine and seven years older than me, and, later, two half-sisters, twelve and eighteen years younger. At various times in my life, I have thus been the youngest child, only child, oldest child, and middle child. What can any researcher really say about that?

Nor have sibling relationships been the same throughout history. Over time, the imprint of culture has shaped the way families behave. From the countless sibling stories in the Old Testament and classical mythology, it's clear that Western civilization has long acknowledged the importance, reward, and difficulty of sibling bonds.

Sibling relationships evolved dramatically through the modern era. By the eighteenth century, historians note, sibling relationships had grown strained by the practice of primogeniture and the rules determining which daughter could marry when. The resulting sibling conflicts began to subside as those laws and customs fell away. The nineteenth-century saw middle-class families embracing the importance of loving relationships within the family, especially between mothers and children. With family money no longer handed down strictly according to sibling birth order, parents began to emphasize loyalty among their offspring instead.

At the turn of the twentieth century, sibling rivalry among the middle class heated back up as families had fewer children and an even stronger focus on maternal-child love developed. No longer steeped in messages of cooperation or required to pitch in to raise the youngest of the brood, children started competing for their parents' love and affection. Sibling relationships continued to evolve in the twentieth century, as closer age spacing and longer time spent in high school meant older siblings were even less able to care for the younger ones. Instead, they were more likely to turn on each other in jealousy. By the late 1900s, the prevailing parenting philosophy led parents to foster a strong sense of individuality in their children via such things as private bedrooms and separate toys.    

Interestingly, it was around that time—the 1980s and 1990s—that the modern science of sibling research took off. In 1996, Frank Sulloway, a psychology professor now at the University of California at Berkeley, published Born to Rebel: Birth Order, Family Dynamics, and Creative Lives, which quickly gained acclaim; Harvard's E.O. Wilson described the book as "one of the most important treatises in the history of the social sciences." It begins with this startling fact: "Siblings raised together are almost as different in their personalities as people from different families." Sulloway wanted to know how this could be. He was particularly interested in creative, revolutionary types, wondering why some people through history are able to reject the status quo to upend societal thinking. Did they owe this capability to age, Oedipal rivalry, gender, random influences—or something else altogether?

Sulloway argued that birth order was the greatest driver of lifelong achievement. Using Darwin's theories of evolution, particularly the survival of the fittest and competition for limited resources, he attempted to show how birth order determines personality, achievement, and adaptability throughout one's life:

In nature, any recurring cause of conflict tends to promote adaptations that increase the odds of coming out on top. In their effort to gain a competitive edge, siblings use physical advantages in size and strength … Over time, the strategies perfected by firstborns have spawned counter strategies in later-borns. The result has been an evolutionary arms race played out within the family.

In other words, once struggling for survival, and now vying for love and attention, siblings compete by developing unique character traits. As in nature, according to Sulloway, children are constantly adapting to get what they need from their parents. This is why the oldest, with assured resources, often cherishes authority, he proposes; the middle child, who can't possibly gain firstborn status, often disengages; and the youngest, nearly lost in the shuffle, seeks the most creative ploys for attention and ends up the family risk-taker. And though other scholars have pushed back against Sulloway's theory (most famously, Judith Rich Harris, in her 1998 opus The Nurture Assumption: Why Children Turn Out the Way They Do), such a scenario gives me some sense of solace; maybe my kids' fighting isn't because of something we've done—it's in their genes. Surveying the chaos on our dining room floor, I start to see my kids less as ill-behaved rapscallions than as a band of Galápagos finches clawing for territory in the family nest.

After our son, William, was born, he, my husband, Michael, and I were a simple triad. As first-time parents, we had lush reserves of time, energy, and resources. Sure, we were slightly panicky in our new roles—William made more first-year trips to the ER than the other two kids have, combined, in their lifetimes—but he seemed to bask in our undiluted attention.

Could we ever love another child this much? According to Jeffrey Kluger, though every parent denies it, "Firstborn favoritism is a very real thing." That's because all the resources we ladle onto our firsts are, in corporate-speak, "sunk costs." Like a business creating its original, flagship product, first-time parents pour so much time and energy into their first child that he will never lose his top spot. We really want him to make it worth our while, and we'll do whatever we can to bolster him so our investment pays off. Then, as the brood grows and we don't have the same reserves of time and energy to dole out, we become less invested, consigning ourselves to being happy with the younger ones for simply breaking even.

The decision to have more than one child—when it is a decision, of course—is personal. In our case, it was born in part from Michael's experience with parents who were both only children. When first his father and later his mother died, Michael and his brother were thrust into caretaker roles for their aging grandmothers. They vowed then that they would each have more than one child. My own family fractured early, and there are wide age gaps among me and my siblings—and a brother who died. I knew I wanted William to have what I never did—a companion of sorts, or at least a sibling who wasn't leaving for college as he was going into fourth grade or being born when he was graduating from high school.

I remember a friend whose family I admired saying, "You have your first child for yourself and your second child as a gift for your first child." I took those words, along with my and Michael's past experiences, to heart. We had that next baby, and then another. And, while our family of five now feels full and complete, I imagine our kids sometimes wish they could return their "gifts." But they can't, and I'm increasingly determined to figure out what, if anything, I can do to make their sibling experience a good one.

"You never say anything when she mimics me," William complains to me one night. "You turn a blind eye, and I can't take it any more."

He's talking about Jessie, who, be it known, cried in my arms that very night because she'll "never be first in our family" then, in the next breath, howled: "I wish I was still a baby but that I didn't fall off the bed or the table." (Because, I suppose, falling off the bed or the table is inherent to the experience of being a baby in our house.) She does have a way of sneaking in her attacks, but they're small and slight—at least to me. To him, they seem to cut like knives.

It wasn't always this way. When Jessie was born, William went through the age-appropriate regression: crying more, sleeping fitfully, and being generally more needy. And then she started to babble and scoot and do outrageous things like slather herself in melted Fudgsicle and make her hilarious "old lady face," and he loved her and it was good. For several sweet years, he tugged her around like a beloved puppy dog. Once, as I pushed them through the aisles of the grocery store in one of those giant blue whale carts, he sang a made-up song: "Jessie is a good little baby, and she will never die!" She followed him, laughing and joining whatever game he devised. She would even try to comfort him when he was upset, toddling over with a stuffed animal or a gentle pat on the back.

But, while she played along when he introduced a game he dubbed "Tackle Jessie," you could see that she was just learning his tricks and biding her time. And then, one day, it was her turn to make a few decisions. The roughhousing and teasing that he'd taught her, she demonstrated with ease. Suddenly, she wasn't such a cute, innocent baby anymore. She was her own person. "Tackle Jessie" became "Tackle William."

And so, for the past year, he has grown increasingly angry, muttering beneath his breath—and worse—when I don't choose to punish her for a perceived (and, in my mind, microscopic) infraction, and taunting her with his big-boy privileges of sleepovers and a later bedtime. (Of course, it could be worse. If I were, say, a black eagle mother with two eggs in my nest, my first chick would have already pecked the other one to death, days after its hatching. Or, take spotted hyenas, wherein a quarter of pups are mauled to death by their siblings. The sad thing is, in our house, chicks used to play lovingly with chicks, and hyena pups once laughed and frolicked.)

My attempts to quell the rising hostility around the dinner table often take dramatic twists, like a series of Hail Mary passes I keep making as my desperation increases. I'll start by trying to channel the How to Talk so Your Kids Will Listen and Listen So Your Kids Will Talk technique. Turning to William, I'd say, "So, you're feeling like Jessie making a weird face at you was unkind?"

When that tactic doesn't work, Michael and I will go old-school on them, pulling privileges like computer time, TV, and dessert. By the end of it all, everyone is usually in a time-out in his or her bedroom. Clearly, we're doing something wrong.

I picked up the Kluger book for insight. I could recognize that something was going a bit haywire in my own family dynamic, but I could also recognize that I was one player in the middle of our unfolding story. The researchers, I hoped, could offer a broader view—one that could help me gain some perspective so I could find a happy ending. Or at least one with less bickering.

It turns out that the influence of birth order is just one corner of sibling research. Researchers have moved beyond how your rung on the family ladder shapes your personal trajectory to explore more complex issues among a variety of sibling configurations. The range of findings is broad, and sometimes leaves me with cognitive whiplash. For instance, according to a study by psychologists Holly Recchia and Nina Howe at Concordia University in Montreal, when parents get involved in sibling conflicts, it takes kids twice as long to resolve the problem, but, with their parents present, they reach compromise slightly more often. (Is "slightly more often" enough of an incentive for me to buckle down and intercede?) I also read that intense sibling rivalry in childhood can lead to difficult relationships later in life, according to Deborah Gold, a psychologist at Duke. (So maybe I should step in and try to squelch it before I miss my chance?) An "unfavored" child is more likely to suffer anxiety, low self-esteem, and depression, according to Clare Stocker, a research professor of developmental psychology at the University of Denver. (So, don't have favorites. Got it.) Having an age spread of four or more years eliminates the issue of competition, says Shirley McGuire, associate professor of psychology at the University of San Francisco. (Really? Cause I'm not really feeling that one.)

Of course, I realize no single study can encapsulate the whole ball of wax. I call Susan McHale, a professor of family studies at Pennsylvania State University, for an in-depth discussion of why my kids may be acting up and if they'll ever get along. She explains families are "comparison machines" that exaggerate children's differences to prevent head-on competition (as, for instance, you might see with those fratricidal hyenas.)

She brings up the idea that launched Sulloway's book—that, in terms of personality, siblings are no more similar to one another than they are to strangers. How can that be?

"Parents think that they're treating their kids the same," McHale says. "And yet, you get these findings. So what is it about the non-shared part of their environment that causes this?" By "non-shared environment," she means anything that multiple kids in the same house experience as being different.

"One of those differences is in the sibling relationship," she goes on. "One has a sister and one has a brother, or one is a first born and one is a later born. And then there's the differential treatment of siblings by the parents."

And that's where we come in. As McHale explains, some in the field believe that the differential treatment of siblings is the most powerful shaper of personality. In other words, you are who you are not just because of how your parents treated you but because of how your parents treated you as compared to how they treated your siblings. Is this why William is getting so upset with Jessie—because he's constantly comparing the way I treat him to the way I treat her? I've long known that I do it, but not in a bad way. It's just that they are such distinct individuals, I can't imagine treating them each in an identical way.

Yeah, McHale goes on, it doesn't help matters. "In order to reduce competition," she says, "siblings de-identify with one another—they consciously or subconsciously pick the niche that's different. So, if you have a smart older sibling, you become the jock, and if your family already has the student and the jock, you become the social butterfly. And if there are no positions left in the family, you become the black sheep."

The kids do this themselves, but—and here's the rub—the parents egg them on. "Parents can be very good at managing this," she says. "We studied sisters who played soccer and we asked them how different they were, and they said, 'Oh, completely different, totally different,' and, 'Well, how are they different?' And the answer was, 'Well, she's on the offensive and I'm on the defensive, and so we're totally different.' So we can see how parents orchestrate this so that their daughters can't compete on how many goals they made that day because one girl's job was to make the goal and the other girl's job was to keep the other team from scoring."      

What ends up happening, McHale argues, is that the family acts as a powerful, too-bright hothouse, based alongside a nuclear power plant. We grow these freakishly dissimilar people so they won't end up eating one another, then wonder why they don't get along.      

Put this way, it makes sense. But what other counter-intuitive insights are out there?

I call Judy Dunn, developmental psychologist at King's College in London, and co-author, with her husband, Robert Plomin, of Separate Lives: Why Siblings Are So Different (1992). She and Plomin found that young siblings are profoundly affected by their mother's interactions with their siblings, and that the little ones notice these interactions from a surprisingly young age.

That little fifteen-month-old or seventeen-month-old, she says, "is watching like a hawk" what goes on between her mother and the older sibling. In one study of this behavior, Dunn noticed that "if either the mother or the sibling showed irritation or anger, the younger child did not ignore it. And the way in which they responded differed, depending on the sort of relationship they had with their siblings. So, if the older child had ticked off the mother by breaking a rule, the younger child would come in and repeat the broken rule and so join the older child in antagonizing the mother." Alternatively, the child might show support for the mother. But, whatever the situation, it's clear—the kids are watching from the get-go. "And the greater the difference in the maternal affection and attention," she argues in her book, "the more hostility and conflict between the siblings."

Starting with the birth of the second child, Dunn and Plomin write, parents can set the tone by minimizing the differences in their relationship with each kid. This, Dunn says, goes against the conventional wisdom doled out in parenting magazines. "A line you see sometimes is, 'Think how you'd feel if your husband said, "It's been so lovely having you, I'm going to have another wife too." ' The implicit advice is not to dominate this first child's life with the baby." But she found the opposite tactic was more effective. In a study of fifty mothers, she found half of them were "those who went out of their way to draw the first child into looking after the baby. So, if he's crying, they'd say, 'Oh why is he crying? Do you think he needs his bottle? I wish we could cheer him up'." The other half took the commonly held advice of keeping the kids separate so they could have full-on parental attention. Guess who fared better.

"On the whole," Dunn says, "the ones who brought the child into the baby's life and vice versa, talked to the first child about the baby as a person with needs and feelings—in those families, the relationship with the siblings did develop more positively."

I like to think I did all that, but as with so much else in those early months with a new baby, I can't precisely recall.

However it all started, I'd like to know how to make it stop. I call Laurie Kramer, a professor of Applied Family Studies and director of the Family Resiliency Center at the University of Illinois at Urbana-Champaign, who was the among the first to quantify sibling conflicts (typically, eight an hour).

Kramer works with parents who want to stop the incessant fighting but don't know how. For her research, she wires kids up with recording devices and leaves them with alternating parents on two separate occasions. The results show that, while parents believe they should intervene in sibling battles, they rarely do so.

"Parents often chose not to respond, and when we looked at the questionnaires we'd asked them to fill out, we found that their confidence was low," she says. "They didn't feel skilled."

I nod so hard in agreement into the phone that my neck starts to hurt. When I describe my own struggles, Kramer replies in a soft, soothing voice.

She explains some of what the Family Resiliency does in teaching siblings to interact with each other. "Maybe your older child needs to get in your middle child's head and vice versa."

This approach sounds good in theory, but I sigh inwardly, imagining another failure.

"Couldn't I just leave them to their own devices?" I ask.

"What we found," she says, "is that when parents do nothing, kids fight more. Especially for kids under age eight, they just don't have the social and emotional competence to work it out themselves."

So, turning a blind eye, as William claims I always do, will not solve this problem.

Armed with the fruits of all this new sibling research, I've started to view our family interactions in a fresh way. I spend thirty minutes rubbing Jessie's back at night, then, exhausted, give William a pat and race off to bed. (Differential treatment.) As I tend to Jessie's tangled hair before school, Annie climbs onto the Thomas table and dances like a wild chicken. (Not Enough Resources, Risk-Seeking Behavior). When we try to get Jessie to take up basketball, she turns up her nose, saying that's an activity for her big, athletic brother (De-identification.)

I also start trying to put the practices that may well work into use. When, for example, a recent dinner was about to implode, I asked the two big kids to try understanding what the other was feeling. It wasn't pretty (I believe the word "nincompoop" was tossed around), and I was glad Laurie Kramer hadn't wired us all up for close examination and Judy Dunn wasn't lurking in the corner with her notepad in hand. But, staying firmly rooted at the dining room table as I attempted to guide them, instead of throwing up my arms in surrender, felt like a small step toward my goal of family peace. The two of them even shared an exaggerated eye-roll afterward at my expense, and our little one made us all laugh by making her own version of "old lady face." It was a rare instant of family harmony—the sort of thing we might one day remember, not in stark detail, but in the warm feeling of belonging that washed over us in that moment and remained, in some intangible way, as night fell around us.

Siblings matter. I've always known that, but I guess I hadn't appreciated just how much and in how many ways. But now, as I watch the unfolding tableaux of my own children's relationships with one another, I think of how all the moments they spend together and all the feelings that crop up, are collecting into the fine dust they will carry with them, whether they like it or not, for the rest of their lives.

Sometimes, at night, I watch William and Annie as they play on our bed. He likes to "babysit" her while I read to Jessie. He carries her up the stairs and bounces her on his lap. Later, when I come up, I see her laughing and squealing as he tickles her. He sometimes tells me she is the only person in the family who understands him. I catch my breath. I fight back tears. I think: What will they give each other in their lifetimes, and how will they break each other's hearts?

It wasn't until after my second daughter, Annie, was born that I realized she and my son, William, share the same seven-year age difference that separated me and my brother, Matt, who died. So, watching them now, I see something I can no longer touch—him and me, growing up. Sometimes I stare, falling out of myself and into the deep chasm of my life's great, unanswered question—not so much "Who am I?" as "Who have I been?" I know the root of the answer lies in our childhood home, with my brothers, one of whom I so rarely see and the other of whom is gone.

I have but one photograph of my brothers and me when I was a baby—we are sitting in a circle, holding hands. My back is to the camera, but I can see their faces, angled toward me and smiling. Other than that small, glossy, black-and-white photo with the thin white border, I don't exist to myself before I began having my own memories. And, once I start to exist, I do so most immediately in the context of my brothers. My parents, dwelling across some invisible bridge in their grown-up world of work and relationships, moved to the periphery of my days and the edges of my consciousness. But there, in the center—in the backseat of the car, in the neighbor's pool, in the garage filled with their motorcycles and my toys—are the three of us doing the complex, fitful, sometimes tender dance of figuring out who we are.

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