Iowa has the greatest racial disparity between black and white people in America when it comes to arrests for possession of marijuana, according to a major new study on drug crime published on Tuesday.

Black Iowans are in fact eight times more likely to be arrested for the crime than white Iowans, the data showed, despite the fact that their rate of marijuana use was about the same.

The American Civil Liberties Union, which carried out an analysis of FBI and census data across the US, said African Americans were 8.34 times more likely to be arrested than whites in Iowa, despite making up just 3.1% of the population. The study showed that black people, nationally, were some 3.7 times more likely to be arrested for marijuana use as whites.

After Iowa, the next largest areas of racial disparity were Washington DC, Minnesota, Illinois, Wisconsin, Kentucky and Pennsylvania.

The news stunned some experts. "The statistics are shocking to me," said L Song Richardson, a law professor at the University of Iowa. Richardson said it was unclear if the police were being actively racist in creating the higher arrest rate, or if the disparity was the result of black Iowans being a visible community in an overwhelmingly white state where federal funding is linked to arrest rates and can provide an incentive to adopt heavy-handed police tactics.

"I am hesitant to say that it is a deliberate racist motive. But we do know that this might be caused by the structural issues linked to policing. Blacks are arrested more, because they are focused upon more," Richardson said.

The ACLU study also looked at the issue of arrests for marijuana on a national level, calculating that between 2001 and 2010 there had been more than 8m arrests for possession of a drug that in many states is now well on its way towards legalisation. It added that enforcing marijuana laws is estimated to cost around $3.6bn a year.

The organisation said law enforcement attempts to curb marijuana use overwhelmingly focused on racial minorities and had the unfortunate impact of ensnaring people unnecessarily in the criminal justice system, potentially costing individuals their jobs and causing widespread social damage for using a drug increasingly tolerated by society. "An arrest for marijuana can interfere with employment when it shows up. That can set people back," said Randall Wilson, the legal director for the ACLU in Iowa.

In recent years, public opinion in the US has shifted markedly towards some form of legalisation of marijuana, whether for medicinal use or for recreation.

Last year, Colorado and Washington state essentially legalised it fully and a total of 18 states have made some form of move towards liberalising its use.

Last month, for the first time in more than four decades of polling on the issue, a Pew survey showed a majority of Americans in favour of legalisation.