A young man who faces the death penalty in the wake of his murder conviction never had the chance to aspire to anything but a life of crime, according to a clinical psychologist.

In testimony Wednesday, Mark Cunningham, a Texas-based psychologist and expert on criminal behavior, cited a dizzying array of statistics and expounded on a “toxic” family tree in order to make some sense of how a teen-aged Rejon Taylor could have reached the point of shooting an Atlanta restaurant owner in the mouth on the side of a Collegedale road.

“You’ve got a generational thing happening that’s almost eerie in his family system,” Mr. Cunningham said, explaining the dynamics of Mr. Taylor’s upbringing to jurors who must decide whether the 24-year-old will be put to death or left in a federal prison for the rest of his life.

With a parade this week of family members, friends and experts such as Mr. Cunningham, the defense is hoping to convince jurors not to put Mr. Taylor to death. The jury’s decision is expected early next week.

Mr. Taylor’s case is notable if only because his trial for the 2003 killing of Atlanta businessman Guy Luck, the man who had become the prime victim of Mr. Taylor’s identity theft scheme, was the first-ever death penalty proceeding to be held in Eastern Tennessee’s federal courts district. Overwhelmingly the domain of state judicial systems, seeking the death penalty for federal crimes is extremely rare and must be sanctioned by the U.S. attorney general in instances of only the most “reckless disregard for human life.”

If jurors sentence Mr. Taylor to death, he will join the 50 federal prisoners now on death row, according to the U.S. Department of Justice. Only three people actually have been executed since the reinstatement of the modern federal death penalty in 1988.

Mr. Cunningham said Mr. Taylor’s regard for his own life, however, already had been significantly marred with a “family script” that included repeat scenes of teen pregnancy, gun use, a total lack of supervision and career criminals as role models.

“We all don’t come to choices from the same level playing field,” Mr. Cunningham said, pointing to research that links childhood experiences to what people do as adults.

Pulling from hours of interviews with family members, Mr. Cunningham talked about Mr. Taylor’s two grandmothers, who became pregnant as unwed teens, and the same behavior in eight other female relatives.

Then there were seven relatives with substance-abuse problems, six male relatives who were absent fathers and still seven others with extensive criminal histories, Mr. Cunningham noted.

One of them was Mr. Taylor’s father, Johnny, who killed someone at 17 and now is in a Georgia prison for large-scale bank fraud, Mr. Cunningham testified. Johnny Taylor would stack wads of cash all around the house, Mr. Cunningham told jurors, and taught both Mr. Taylor and his brother how to use the numerous guns hidden between books and under pillows in case anyone ever tried to rob them.

Although Mr. Taylor’s mother did not participate in his father’s fraud, she accepted the money from it, Mr. Cunningham said, never disciplining her children or telling them that it was wrong.

At one point, Mr. Taylor’s father even questioned the mother’s parenting skills, Mr. Cunningham said.

“Here’s a life-long career criminal, and he’s concerned about the lack of supervision in the home, which is pretty startling,” Mr. Cunningham said.

Jurors convicted Mr. Taylor of murder, kidnapping and carjacking in mid-September after three weeks of witness testimony. They were swayed by evidence that strung together a story of three teens who started out as mailbox robbers in Atlanta’s wealthy Buckhead neighborhood but would end up interstate kidnappers and murderers.

“I didn’t think I was hurting anybody,” Mr. Taylor told jurors earlier this week as he described the identity-theft lifestyle he said he inherited from his father. Mr. Taylor continues to maintain his innocence in the killing.

Yet Mr. Taylor’s two friends, Joey Marshall and Sir Jack Matthews, pleaded guilty in 2006 for their role in the crime, and prosecutors showed forensic evidence that pointed to Mr. Taylor as the one who had fired the fatal shot.

The same day as Mr. Luck’s death, Mr. Taylor, 19 at the time, and one of his accomplices treated their girlfriends to a dinner at a Red Lobster in Atlanta, using money they had stolen from Mr. Luck to pay for the meal.