After he was shot in the leg by a fleeing convenience store robber, Mark McManus was asked why he chased down someone who was clearly dangerous.

His answer? It didn't occur to him not to do it. The part of McManus' brain that evaluates risk doesn't work.

The 49-year-old Billings man's brain was damaged by exposure to alcohol in the womb. He has fetal alcohol spectrum disorder, or FASD.

"You want to do the right thing, and it was the right thing to do," McManus said of following Thomas Servais from the Kwik Way store Servais robbed in June 2005.

McManus was driving along Grand Avenue when he saw Servais leaving the convenience store across the street from Senior High.

A lifetime of working overtime to understand social cues - people with alcohol-related birth defects struggle to fit in - helped him realize something wasn't right about Servais.

Servais got into a car, and McManus followed him. A few blocks away, Servais whipped his vehicle around and shot McManus in the leg. Minutes later, Servais was killed in a shootout with a deputy sheriff.

Some people hailed McManus as a hero - he alerted authorities to the robbery - and others said he was foolish for getting involved.

Either way, he did what he did because prenatal exposure to alcohol affected the way his brain is wired. Being unable to understand cause-and-effect relationships or the potential consequences of actions is a hallmark symptom of FASD.

McManus knew all his life that something was different about him. He excelled at tasks that involved verbal skills, but he flubbed anything that required abstract thinking.

"It's like somebody is teaching something on the blackboard in Chinese," he said. "When you don't get it, they speak louder."

He was diagnosed with FASD about eight years ago after his puzzling shortcomings began affecting his job. His work duties had changed to include data management, and no matter what he did, McManus could not understand the charts and grids involved in his new assignment.

His wife, Amy, a nurse, was the one who suggested his problems might be linked to fetal alcohol exposure. The FASD diagnosis was at once a relief and a devastating blow, McManus said.

He did not like being told that there were some things he simply would not be able to do because of the damage to his brain. But after a period of grieving, he rallied.

"If you have no legs, you're probably not going to be a square dancer," he said. "So, what can you do? If you can't go this way, go this way."

McManus works as a chaplain at Rimrock Foundation, where he was treated for alcoholism two decades ago, and at the police department.

His mother died 19 years ago. She was sober by then, but her years of drinking caused her early death, McManus said.

He was at her side when she died and has forgiven her for the hurt her drinking caused him.

"Being an alcoholic myself, I understand how that can get so insanely out of control," McManus said. "Everything that needed to be said was said."

Contact Diane Cochran at dcochran@billingsgazette.com or 657-1287.