The nongovernment human rights group says it amounts to a caste system.

Despite the emergence of informal markets since the late 1990s challenging state control, the study says the class system persists and is behind the discrimination and abuses faced by the lowest echelons of the North's closed society.

"Throughout its 64-year existence, the Kim regime has claimed that North Korea is an egalitarian workers' paradise," said the committee's executive director, Greg Scarlatoiu. "Yet inequality is assigned at birth, perpetuated throughout a person's lifetime and cruelly enforced by those in power to benefit themselves and their supporters."

The North Korean government denies such a discriminatory class system exists.

The study, titled "Marked for Life," is based on interviews with 75 North Korean defectors, including as recently as 2011. It also cites a 1993 manual issued by North Korea's Ministry of Public Security to guide its officials on how to investigate a citizen's socio-political classification, or "songbun," which translates in Korean as "ingredients."

The ministry maintains a file on everyone from the age of 17 that is updated every two years, the study says.

Numerous defectors' interviews show that those who are young when they leave North Korea see songbun as decreasingly important, while older defectors say songbun still matters, the report said.

The songbun system has its origins in social class restructuring enforced by the North's communist founders, led by Kim Il Sung, that began even before the state's formal creation in 1948, to elevate peasants and laborers at the expense of landlords, businessmen and religious leaders.

Those considered most loyal had fought alongside Kim against Japanese colonialists and then against U.S.-backed forces in the 1950-53 Korean War. Those deemed hostile collaborated with the enemy or had family members who fled to South Korea.

Moving up a songbun category is rare and requires a lifetime of devotion to the Kim family regime and ruling Workers' Party of Korea. But songbun can be downgraded for political or criminal offenses or failing to cooperate with authorities.

When an individual is sentenced to the North's gulag of political prison camps -- estimated to hold 150,000-200,000 people -- family members are considered guilty by association and generally accompany them, the study says.

Today the loyal class, which makes up about a quarter of the 24 million population, still dominates the powerful military and the Workers' Party. They alone are entitled to live in the relatively prosperous capital Pyongyang and monopolize the prestige universities and best jobs, the study says.

Marrying someone with poor songbun likely would exclude that individual from party membership, causing severe consequences for employment opportunities and quality of life, it says.

Many in the hostile class inhabit the most impoverished northeastern provinces, often in isolated mountain villages where they perform hard labor at mines and farms. They have been most vulnerable to failures since the 1990s in North Korea's public food distribution system and resulting malnutrition.

Roberta Cohen, co-chairman of the rights group, says money, bribery and corruption recently have begun to erode the songbun system because of the emergence of informal markets and the scope for paying officials for favors. But she said songbun's main elements remain in place, "guaranteed by a complete absence of political freedom."

The study says slang has developed around the songbun system, wherein those in the loyal class are referred to as "tomatoes" that are red on both the inside and outside, so are good socialists. Those of the wavering class are "apples" that are only red on the outside, and the hostile class is known as "grapes," considered politically unredeemable.

There's no sign that new ruler Kim Jong Un, the third in the dynasty, will change the policy. The study concludes that would be a direct threat to the North Korean elite and could undermine his consolidation of power.