When Drew Harrison was in prison, just sitting in his cell overloaded his senses. To dim the fluorescent lights, he covered the bulbs with toothpaste or paper. To mask overwhelming odors, he wrapped his uniform around his head.Once, when he asked to stay longer in the prison yard, Harrison, who has autism, said he was told he was being insubordinate and was put in restrictive housing. Another time, he said he requested placement in solitary confinement so he wouldn’t have to interact with people, which he found stressful.After two and a half years, Harrison, who had been convicted of sexually assaulting an ex-girlfriend, thought he would get out more than a month early from Greensville Correction Center in Virginia, because of good behavior. But because he had failed to sign up for a job while incarcerated earlier at another Virginia prison, he had forfeited his chance at early release, according to documentation from the Virginia Department of Corrections.Harrison’s autism made it difficult for him to grasp the rules, his mother, Judy Harrison, explained. Only after she lobbied a Virginia state legislator was her son’s “good time” reinstated; he was released in 2019. “I don’t know how someone with Drew’s diagnosis would have survived in prison without an advocate,” she said. “You can’t teach someone not to have autism.”The prison environment—with its loud, unpredictable noises, bright lights, unpleasant odors, complicated social dynamics and often byzantine bureaucracy—would be difficult for anyone to navigate, but experts say that it’s particularly challenging for autistic people and people with similar disabilities.The Americans with Disabilities Act—signed into law 30 years ago this summer—mandates that people with physical and developmental disorders receive equal access to programs and services provided by public institutions, including correctional facilities. But advocates for people with developmental disabilities have long argued that all too often, prisons do not fulfill that promise.One reason may be that many states don’t adequately identify prisoners with developmental disorders. The Marshall Project sent questions to all 50 state corrections departments asking whether and how they screen prisoners for developmental or intellectual disabilities. Of the 38 agencies that responded, 25 reported using screening protocols that several mental health and legal experts said don’t meet professional standards. Five states said they don’t screen for developmental disabilities at all.When developmentally disabled prisoners go unidentified, they are even less likely to receive services they are entitled to under federal law—such as help understanding prison rules or obtaining medications. That loss of assistance leaves them vulnerable to medical misdiagnosis, isolation in solitary confinement, denial of legal and educational opportunities, sexual abuse and bullying, prisoner advocates and relatives say.“It mirrors or echoes what goes on in society. People with disabilities are often hidden and not seen,” said Susan Politt, a supervising attorney at Disability Rights North Carolina, an advocacy agency. But behind bars, the situation can be even more dire. “They get out in worse condition than when they go in,” she said.Experts say that incarcerated people of color with intellectual and developmental disabilities are affected disproportionately because they are less likely than their White counterparts to be diagnosed before they enter prison.Developmental disability is an umbrella term for a group of disorders that start during childhood and restrict one’s ability to learn, speak, behave or physically develop; they include autism, cerebral palsy and language and speech disorders. The most common type is intellectual disability. It is generally accepted that 2 to 3 percent of people around the world have an intellectual disability, said Maggie Nygren, executive director of American Association on Intellectual and Developmental Disabilities.The proportion of people with developmental disabilities behind bars is likely to be higher than in the general population, research suggests. The Arc, a nonprofit that advocates for people with disabilities, estimates that between 4 and 10 percent of U.S. prisoners have an intellectual disability, a range supported by recent literature. In 2015, the Bureau of Justice Statistics reported that about 20 percent of prisoners mentioned having a cognitive disability, a category that includes developmental disability.But when The Marshall Project contacted state corrections departments, only four of the 25 states that shared their 2019 numbers reported that the population of prisoners with developmental or intellectual disabilities was in that range. The other 21 states that shared their population numbers said that prisoners with developmental disabilities made up less than 4 percent of the total prison population.One disability rights lawyer characterized those low numbers as implausible. “There’s no rational basis under which a prison population should have a lower percentage than a general population,” said Bill Van Der Pol, a senior staff attorney at the Alabama Disabilities Advocacy Project, who won a class-action lawsuit in 2014 on behalf of physically and developmentally disabled prisoners.Thirteen states told The Marshall Project that they did not track the population numbers or could not provide them.