HAVERHILL — Statistics show the overwhelming number of murder-suicides involving husband and wives, or boyfriends and girlfriends, are carried out by the man — and almost always involve the use of a gun.

Organizations that collect such information say they know far less about what motivates a woman to commit murder-suicide than what pushes a man to such violence.

But that is just what happened in a car near Winnekenni Castle on Wednesday afternoon, when Martha McDermott, 36, shot her husband, Kristin Broderick, 37, to death and then committed suicide, investigators said.

According to experts who study cases of murder-suicide, a woman killing her husband and then herself happens far less frequently than a man carrying out the violent act.

"The only usual aspect of this incident is the use of the gun," Kristen Rand, legislative director of the Violence Policy Center, said of the incident in Haverhill, which has left the community shocked and confused.

'A very unusual case'

Relatives and friends of McDermott and Broderick are refusing to talk about what might have led to them meeting at the site and then McDermott killing them both.

"Kris was a great kid and Martha was a great girl. I'll never understand it," Broderick's father, Philip Broderick of Haverhill, said the morning after the killings.

Rand said it is much more unusual for a woman to commit a murder-suicide, so it's often harder to explain than when a man is the perpetrator.

"We don't have enough cases to really pinpoint a common thread. And it is very unusual for women to kill their spouse or intimate partner, so this is a very unusual case," she said.

Rand said one of the first factors to look at in any murder-suicide is whether there is a breakdown in an intimate partner relationship. The other factor is a custody dispute, which is a common factor in a lot of murder-suicides, she said.

"Another thing to look at is there any history of depression, such as postpartum depression," Rand said.

McDermott and Broderick were married in June 2003. Broderick's Facebook page tells a story of a happy relationship — from photos of him and his wife hugging and smiling to the announcement of the March 2011 birth of their son, Clayton Edward Broderick.

Police and relatives have said the child is safe with family members.

Rand said her organization has conducted three studies on the phenomenon of murder-suicides, the last in 2008. Each study showed consistencies in terms of the perpetrator, the kind of weapon used and where the incidents tend to occur.

Rand said that in general, most incidents of this kind are committed by men because men are more prone to violence.

"In cases of murder-suicides, the triggering event is usually some sort of stress event, which involves a loss of control such as a breakdown in a relationship or serious economic problems," Rand said. "Men are less likely to handle a stress event and are less likely to seek help."

Although investigators believe the couple were estranged, police refused to answer questions about how long they may have been apart. Most recently, McDermott was living in a condominium at 180 Drakeside Road, Hampton, N.H. Her husband is believed to have been living in Houston, Texas, and had a Texas driver's license, according to police.

Broderick was a geologist who had worked in Houston and Angola. According to his obituary, he traveled the world for work and pleasure and recently returned from a three-year stay in Angola. He was employed as a geologist for the Exxon-Mobil Corporation in Houston for the past seven years. He grew up in Haverhill.

According to their wedding announcement on seacoastonline.com, McDermott and Broderick were married in June 2003 at St. Andrew's-by-the-Sea in Rye Beach, N.H. McDermott graduated from Emerson College in 1995, from the University of New Hampshire in 2000 and from the University of Redlands in 2003. She was employed as a speech-language pathologist in Santa Barbara, Calif., where she lived with her new husband. Kristin Broderick graduated from Haverhill High School in 1992 and attended Emerson College. He also attended Salem State College and the University of California, Santa Barbara.

Study: 95 percent of murder-suicides done by males

According to the third edition of "American Roulette" — a study on murder-suicides in America that was published in 2008 by the Violence Policy Center in Washington, D.C. — murder-suicides usually involve a gun and a man killing his wife or girlfriend because of a breakdown in their relationship.

The Violence Policy Center is a tax-exempt educational organization that provides information to policy makers, the media and the public, and has a focus on reducing gun violence in America.

According to the latest study, 88.5 percent of murder-suicides involved a gun and 95 percent of the incidents were carried out by males. The study shows that for intimate partners, 80 percent of murder-suicides occurred in the home, and more often in the bedroom than any other room.

Why are guns so common in murder-suicides?

"Firearms allow shooters to act on impulse," the report said. "Every major murder-suicide study ever conducted has shown that a firearm — with its unmatched combination of lethality and availability — is the weapon most often used to murder the victims, with the offenders then turning the gun on themselves."

The report suggests that efforts be made to restrict access to firearms where there is an increased risk of murder-suicide, such as in cases where an individual has a history of domestic violence or has threatened suicide.

Police said the bodies of McDermott and Broderick were found shortly after 1 p.m. Wednesday in a silver Volkswagen in a parking lot at Winnekenni Castle, located in a wooded park of several hundred acres where people go hiking and biking. The bodies were discovered by people walking in the park and police were called.

The state medical examiner performed autopsies on both bodies last week. On Friday, the Essex District Attorney's Office said details on the cause of death will be released this week, following additional testing by the medical examiner.

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