A police officer stands guard at the scene as the...

A police officer stands guard at the scene as the search for suspect in the Boston Marathon bombings continued in Watertown, Mass. (April 19, 2013) Credit: AP

My Twitter feed has been flooded with this theme: The Tsarnaev brothers seem more like the Columbine killers than al-Qaida.

Maybe. Either, neither or it could easily be a combination. It's way too early to know. The first thing I learned covering Columbine all those years was that most of the theories that gain traction early will be wrong.

But we do have an interesting situation developing with a pair of brothers as suspects: potentially, the classic dyad scenario. Notorious dyad examples include Bonnie and Clyde, Leopold and Loeb, and the D.C. snipers. The dyad tends to be a twisted, particular relationship that plays out very differently than the lone gunman or the terrorist team.

Since that idea is getting a lot of attention, let's explore the "dyad" phenomenon and how dyads typically play out. Whether that pattern was relevant here will be determined later. We could be looking at a fusion situation, where Tamerlan Tsarnaev trained as a traditional terrorist, and followed the dyad model in the way he integrated Dzhokhar.

We don't even know whether this was a true dyad in the sense of joint planning. For all we know, Dzhokhar (or Tamerlan) learned the contents of the duffel bag only 20 minutes before the attack. His older brother could have asked him to carry the bag for him and drop it over there. Dzhokhar could have suspected a little, a lot, or anything in between. All we know is that they were both there at the scene - and then became fugitives together. What came before is all still conjecture.

The good news: There is a typical dyad pattern. That's in contrast to lone killers, who run the psychological gamut. Every study has drawn the conclusion that there is no typical mass murderer. Mass killers are mostly not loners or outcasts, and the Columbine killers were neither.

Killer dyads are more consistent. And the popular conception of the dominant, charismatic leader roping a submissive follower into his diabolical scheme - surprisingly, that usually turns out to be true. The leader is commonly a sadistic, dehumanizing psychopath - not always, but far more often than is the case with lone gunmen/bombers, where that personality type is relatively rare. The follower is often depressive, submissive or otherwise dependent.

When there is a significant age difference - as with one killer just out of high school - we can't be certain the older partner plays the lead role, but it usually works that way.

Dyads usually contain contrasting personalities. A psychopathic killer generally does not link up with another psychopath. Nor do depressives pair up. Thrill-seeking psychopaths have been known to pair up, but most are looking for the qualities they lack.

Here, Columbine is highly illuminating. It's a lousy example for understanding most school shootings, because it's so atypical: It wasn't even intended primarily as a shooting - the main event was the failed bombs. But Columbine is a perfect illustration of the classic dyad: Eric Harris wanted a minion to march behind him; Dylan Klebold was looking for someone to lead a parade.

Psychopaths like Eric Harris crave excitement and have difficulty sustaining it. An ambitious killer may crave a pliable, excitable assistant. He is not in the market for argument, criticism or someone to share the glory. A No. 1 fan would be super.

Dylan was unlikely to hook up with another kid wallowing in self-misery. Eric radiated confidence, charisma, and the vision of a glorious bloody way out.

I have often wondered why Eric even recruited a partner. Their writings indicate that Eric sought out the arms, collected the ammo, researched the big bombs, built all the pipe bombs, drew up the plans and diagrams, conducted the reconnaissance, calculated how to maximize the body count, cooked up batch after batch of failed napalm, and generally devised the plan. What exactly did he need Dylan for?

To carry one of the duffel bags and shoot additional people? Couldn't Eric have coaxed his buddy into dropping a duffel bag in the cafeteria without mentioning the propane bomb inside? Didn't he have enough firepower with his two guns to kill hundreds? He could have killed far more than 13 people if he focused on work instead of laughing it up and gabbing with his partner the whole way through.

Which is the nub. The shooting was superfluous anyway - it was to be dwarfed by the bombs. The shooting was supposed to be the fun part. "Have fun," they wrote on the schedule for their last act.

Almost 14 years to the day after Columbine, I would say Dylan's main purpose in the whole tragedy was for Eric to have fun. What's the fun of a shooting spree on your own? And more importantly, how does it entertain you the entire year leading up to the attack?

Serial killers don't space out their murders for efficiency. They do it to maximize enjoyment. Death and torture are the amusing parts. They want to relish the screams over and over. They want recognition. They sometimes assist the police investigation, not to get caught, but to toy with the cops, and to sneer at them.

Sadistic killers who go the event route, as with Columbine, need to plan for months, and they are hungry for satisfaction during that planning period. When you only get one bomb blast, the thrill of the plot is everything. Dylan offered Eric a highly intelligent audience of one. They were laughing for month after month at the fools missing the plot unfolding right under their noses.

The boys videotaped themselves at an afternoon of target practice with several friends six weeks before the attack. They shot up a bowling pin and then a tree trunk, "Imagine that in someone's fucking brain," Eric said. They all laughed, but only he and Dylan got the real joke.

Months it went on. An entire year of satisfaction. None of that would have been possible without one hapless follower in on the joke.

That's how dyads typically operate. Whether that model ends up fitting the Boston massacre is something we may discover soon.

Dave Cullen is the author of "Columbine."

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