Cases in which a juvenile is accused of killing both parents are exceedingly rare.

Dr. Kathleen Heide, a professor of criminology at the University of South Florida who has studied parricide for 30 years, said in a phone interview Tuesday that cases such as the Winthrop killings, in which a juvenile is accused of slaying both parents, represent less than 10 percent of the roughly 250 murders of parents by their offspring reported annually in the nation.

Emphasizing that she has no specific information about the Winthrop murders, Heide said that in most parent-homicide cases, the offenders fall into four categories: the severely abused child, the severely mentally ill child, the dangerously anti-social child and the enraged child. The categories apply both to adults who kill their parents and juveniles who do so, she said.

Severely abused children may kill not out of retaliation, but out of fear or desperation for their own lives or the life of another family member, or out of a desperate sense that killing is the only way out of an abusive situation, which is frequently well-documented.

In cases of severe mental illness, offenders typically have a formal diagnosis of mental illness, a loose relationship to reality and may often engage in delusional thinking, believing, for instance, that the devil is speaking to them or directing their actions. Offenders of this type who kill their parents are most often adults, she said.

The dangerously anti-social offender, on the other hand, kills a parent or parents out of intensely selfish motivations, including murders motivated by money or by the child seeking to gain freedom or privileges that the parents, when alive, would refuse to grant.

“It’s as if the parent is an obstacle and they want to get the parents out of the way,” Heide said.

Enraged offenders kill because of pure rage, possibly borne from abuse or neglect, or from other factors such as the sudden imposition of limitations, or a sharp removal of financial or other types of support. In cases of rage, Heide said the offender is often, but not always, influenced by drugs or alcohol.

She said the two most common defense strategies of juveniles who kill their parents are an insanity defense or a battered-child-syndrome defense, in which the defense may argue that the killing was the result of years of extended abuse. Both are seriously challenging, and have low chance of succeeding.

“Typically what happens in these cases, the juvenile is almost always prosecuted as an adult, typically charged with murder in the first degree or murder in the second degree, and many times these cases plead out,” Heide said. “Many times they’re convicted of second-degree murder or manslaughter. So it’s very rare that the juvenile is exonerated.”