"History and drama are full of examples of the favored prince who got into the world and couldn't function because he was not accustomed to an environment where he was judged more objectively," he said. "They tend to get a real waking up and a humbling in the world."
Favorites also take more heat from their siblings, according to Kluger, which can make home life miserable.
Kluger also says fighting is nature's way of competing for food and our parents' attention. "We are also genetically driven to show off our strength and general fitness -- another way to ensure that our parents will love us and look after us," he said.
Not surprisingly, even well-adjusted children ages 3 to 5 will have up to seven fights an hour.
Kluger also addresses the much-maligned only child. Not only are they smarter, but they are more self-sufficient and have more "sophistication," because of all the adult company. With modern day care and play dates, many get just as much socialization as children with siblings.
"They are living in a world in which no one cares about picking sides, where there is no favorite," he said.
Research on birth order remains constant. Firstborns are the highest achievers with IQs that are on average 1 to 3 points higher than the next sibling. In addition to being physically larger, they also become the biggest earners.
"Younger siblings tend to be the charmers, the adventurers, the comedians and the rebels," said Kluger. "Middle siblings take longer to find their identity."
They grow up at the mercy of their older siblings and experience much of what Kluger's baby brother Bruce had to endure.
"My brother force fed me caterpillars when I was little," said Lisa Palmer, now 61 of Glastonbury, Conn. "My mother told me that he never got over having to share her with me."
Jeannie Pierce, 57, and the younger of two sisters from Shrewsbury, Mass., said her sister pushed her carriage down an incline and the 6-month old fell out and suffered a concussion.
"She also stuck gum in my hair so my mother had to cut it real short," said Pierce. "She also locked me in the cellar during a thunder and rain storm, during which I cut my wrist open banging on the door window."
Much of the turmoil, just as in the Kluger household, was carried on under the radar of their parents.
Pierce's jealous older sister, while charged with babysitting, left her 8-year-old sibling at home so she could go to a dance. She also chose the bed by the window and made her little sister sleep by the door.
"Her rationale for this was that, if someone broke in and tried to murder us, she would be able to escape out of the window while I was being murdered," said Pierce.
One older sister is quick to confess that she made her younger sister's life hell. "Oldest siblings eventually know the truth," said Susan, a 61-year-old first-born from Portland, Maine. "We are all recovering tyrants."
"The family story goes that when our parents told me that they were so excited that they were having another baby, I said that that sounded great as long as we could 'get rid of that one,' my sister Laura."
Kluger's research affirms that middle siblings may not get showered with the same adoration, but they often have better relationships and more independence as they grow older.
They are also more empathetic.
"We learned from you," Laura, 59, from Oakland, Calif., said of Susan and other first-borns. "And then we set that aside and took our own places, informed by what we had seen... Today, we are about the same age and we face our mortality and our wrinkles and losses and our looming old age together."
Not all siblings end up as friends. One large study revealed that about 15 percent have relationships that are beyond repair, "broken and badly fraught."
At the heart of Kluger's message is that as we age, strong sibling relationships can keep up healthier mentally and physically and perhaps even slow down dementia. A brother or sister may even be the one to look after us.
"The longer life expectancies get, the more of us will arrive in an old age in which we've outlived a spouse and other loved ones, and our kids have scattered," said Kluger. "Sibs are often the only ones left -- and often the people who know you and love you the best."
He continues close ties with his three brothers and two half-siblings through e-mail, regular phone calls and Skype.
"Another thing that struck me in a positive way was blended families," he said. "Six years is the threshold point past which step-siblings become virtually indistinguishable from whole siblings. In some ways, they are closer because they don't have early life competition."
But old rivalries can persist, even decades later, especially at family gatherings.
"Someone once said, 'When I go home for Thanksgiving, so often the meals turn into old family wars.' Of course, people push the buttons – they installed them," said Kluger.
As for Bruce, he was always his mother's favorite, "in part because he was my father's least favorite and came in for occasional harsh treatment as a result," remembers Kluger, who always felt "a bit in the shadow" of the bespectacled redhead with the "incandescent personality," even well into adulthood.
A decade ago while visiting their mother, Kluger and his Bruce unconsciously regressed to childhood behavior. "We were walking toward her car and we both picked up the pace," he said. "We were racing to get the front seat."
Both boys have retained a sense of humor about the hours Bruce spent latched in the cabinet in the basement. After Kluger's book came out, Bruce Photoshopped a plaque on an old picture of their house: "6026 Berkeley: Home of the Fuse Box."
"We saved his life," said Kluger, only half-kiddingly.