(CNN) -- ln April, a Maryland man wrote six suicide notesexpressing his love and sorrow for his family, and then shot his wifeand three children, before killing himself with a shotgun.

Curt Wheat and his wife, Marie, married in 1971. After 32 years, Wheat shot his wife and then himself.

This month, a man killed his wife and two sons in their home nearTampa, Florida. Troy Bellar chased after his teenage son with ahigh-powered rifle, but the 13-year-old escaped. Bellar later shothimself.

In some of this year's most disturbing cases of familyviolence, fathers have turned against their own flesh and blood --asphyxiating and beating teenagers, firing shots into sleeping childrentucked in bed, slaying grandparents and shooting infants in diapers.

The killings are cruel inversions of nature where a father murders hisentire family in an act called familicide. After the carnage, thequestion lingers: Why did they do it?

For decades,psychiatrists have been studying such cases to determine what mentalissues trigger this behavior. A person who kills his family could havecontrol issues that lead him to decide the fate of the children, spouseand pets, researchers said. See a timeline of 2009 familicides »

While mentally healthy people cannot make sense of killing someone theylove, for people with mental illness, "it has to do with theirdistorted thinking and depression," said Donna Cohen, a professor andhead of the Violence and Injury Prevention Program at the University ofSouth Florida.

The person with a mental illness views his wifeand children as possessions, believing, "I have to keep this. This ismine," Cohen said. "Nobody else is able to take care of them except me.If I can't control this in my life, I'll preserve it in death so thatmy world doesn't change. It's the psychiatric issues."

Murder-suicide plots could brew in a person's mind for months, even years.

Fact Box Out of 1,500 to 2,000 murder-suicides a year,the Violence and Injury Prevention Program at University of SouthFlorida estimates

70 percent have two victims

10 percent are familicides

7 to 8 percent involve children and babies

7 to 8 percent involve nonfamily members

2 percent are mass events "For a period of time, the idea to commit mass murder takes hold ofthem and they can't shake it," said Louis Schlesinger, forensicpsychology professor at John Jay College of Criminal Justice in NewYork City.

If there is news about other familicides, the intensenews coverage may cause others to lose their inhibition and commit thesame type of crime.

"They think they're saving their family and that they will be remembered with sympathy," Schlesinger said.

Those thoughts become action when a precipitating event, or trigger,occurs, such as divorce, suspected infidelity or loss of job.

While research shows that suicide rates rise when the economy is in trouble, it may not be true for murder-suicides. See graph on the economy and suicides. »

Statistics from the Violence and Injury Prevention Program at theUniversity of South Florida based on news reports, found thatmurder-suicides seesawed during the last nine years.

Accordingto research at Case Western Reserve University in Cleveland, Ohio,about 60 percent of the perpetrators of familicides were unemployed.While the shaky economy could increase stress levels, experts warnedagainst attributing the spate of violence to the economy.

"That's careless to say a bad economy causes people to kill wholefamilies," said Schlesinger. "You could kill during good times. It'svery complicated and not simple as a bad economy."

Don't Miss UGA prof dug grave, shot himself in head, police sayFlorida man kills wife, 2 kids, himself, officials saySuicidal thoughts, high blood pressure associated with insomnia In research during nearly three decades, Dr. Philip Resnick, directorof the division of forensic psychiatry at Case Western, has outlinedtwo motives for familicides: revenge or despondency. In latter cases, a"nonhostile, hopeless father" kills his family to save them fromperceived doom, because he feels unable to provide for them.

"They become very depressed as the breadwinner," Resnick said. "Withtheir distorted, depressive perceptions, they feel that rather thanallow their children to go hungry, they may feel they're doing a favorto take their family with them as they end their own life. ... They'renot depriving them of life, they're ending what they see as anintolerable life."

In recent cases, some men were facingfinancial turmoil. In April, a New York attorney who was involved inquestionable financial dealings asphyxiated and beat his wife and 11-and 19-year-old daughters in a Maryland hotel. Investigators laterfound that $20 million from his clients was missing.

A man who decides to commit suicidemight want to avoid his family facing the stigma, said Richard JamesGelles, dean of the School of Social Policy and Practice at theUniversity of Pennsylvania.