BISMARCK, N.D. — For most of his career, North Dakota demographer Richard Rathge watched an extended jailbreak-like exodus of residents from the frigid and once forlorn state. North Dakota now is leading the nation in population growth and the number of residents in the state is at an all-time high, thanks to an oil bonanza in the western part of the state.

Rathge, 62, a professor of sociology at North Dakota State University in Fargo, is retiring next month after 32 years of tracking population and economic trends in the state. The New Mexico native came to North Dakota in 1981 and plans to retire to the Twin Cities area of Minnesota.

“I moved to North Dakota when no one was coming here and now everyone is coming here and I’m leaving,” said Rathge, whose wife, Polly Fassinger, is the new director of institutional research at Macalester College in St. Paul.

The turnaround in the state’s fortunes came “purely from oil” beginning in about 2006, reversing a seven decadeslong trend of outmigration, where more people were going than coming, Rathge said.

“For years, I was the fall guy who carried that dark cloud of bad news that no one wanted to hear,” he said. “Half of North Dakota’s 53 counties had persistent decline since 1940. It’s been decline, decline, decline and all the sudden, boom!”

North Dakota’s population had peaked at 680,845 in 1930, and was surpassed only in 2011. The state has a population of more than 700,000 at present but still ranks 48th in total population, ahead of only Vermont and Wyoming, Rathge said.

In 2003, a decade-low 632,809 people lived in North Dakota, the only state to record a loss of population for the year.

Especially alarming over the years was North Dakota’s loss of young people between the ages of 25 and 39, which from 1995 to 2000 was the highest outmigration rate of that age group in the United States, the Census Bureau said.

Tired of a yearslong talent suck, the state sponsored job fairs across the country as late at 2007 to lure people — especially former North Dakotans — to the state.

Young adults are now relocating to North Dakota or staying because of a strong economy, Rathge said.

The state has some 21,000 more jobs than takers at present and the lowest unemployment rate in the nation, at less than 3 percent, Job Service North Dakota data show.

Until last year, Rathge headed the state Data Center and was the state’s official demographer. The state Department of Commerce now compiles census data.

Kevin Iverson, manager of the state’s census office, said Rathge’s research over the years continues to be invaluable.

“His depth of knowledge is just exceptional in terms of interpreting what’s happening in North Dakota,” Iverson said.

Rathge also serves as the policy analyst for the North Dakota Kids Count program, which ranks states in economic well-being, education, health, and family and community.

“He has a real heart for people and his work reflects that,” Iverson said.

The state Commerce Department continues to rely on population projections that have been done by Rathge, who estimates the number of state residents to balloon to 850,000 by 2025.

Rathge said North Dakota’s strong economy and soaring population also has created a “tremendous contradiction,” with increases in such things as crime and homelessness.

“There is an increase in poverty, and between the haves and haves not,” Rathge said.

“This boom will go bust,” he said. “We are sitting on billions of dollars and we must invest in a sustainable economy that does not rely on oil and gas or we will be back in the same position were,” he said.