Robert Downey Jr with his son Indio at a basketball game in 2008. Photo: Getty
Actor Robert Downey Jr has thanked US authorities for arresting his 20-year-old son on suspicion of cocaine possession and says the family is determined to get him the help he needs.
Indio Falconer Downey was taken into custody on Sunday afternoon after a car he was riding in was pulled over in West Hollywood, authorities said.
"Unfortunately, there's a genetic component to addiction and Indio has likely inherited it," Robert Downey Jr, who had a series of high-profile drug-related arrests in the late 1990s, said in a statement.
"We're grateful to the Sheriff's Department for their intervention, and believe Indio can be another recovery success story instead of a cautionary tale."
Downey Jr, whose own father was also a drug addict and alcoholic, is not wrong.
According to the National Council on Alcoholism and Drug Dependency (NCADD), "the single most reliable indicator of risk for future alcohol and drug problems is family history."
The NCADD adds: "Research has shown conclusively that family history of alcoholism or drug addiction is in part genetic and not just the result of the family environment."
They point out however that genetics are only part of the picture. It is nature and nurture that determine outcomes.
Scientists are still trying to understand the interplay of factors. In a paper, by Australia's Ministerial Council on Drug Strategy, the authors state that: "Despite the strong evidence of genetic contributions to addiction vulnerability, attempts to reliably identify specific addiction susceptibility genes have been disappointing to date."
Robert Downey's drug use sent him to prison and rehab and cost him acting roles.
In one of his many court appearances, he told a judge: "Taking drugs to me is like having a shotgun in my mouth with my finger on the trigger, and I like the taste of the gun metal."
He didn't get serious about getting clean until another stint in rehab in 2003; Indio was 10 by that stage.
Downey is now one of the industry's most bankable stars after appearing in Marvel's Iron Man trilogy and The Avengers.
Indio Downey was booked on suspicion of possession of a controlled substance, believed to be cocaine, and possession of drug paraphernalia, sheriff's Sergeant Dave Valentine said.
He was later released after posting $10,250 ($A11,090) bail.
A court date was set for August 29.
Two other people were in the car, but only Downey was arrested.
He appeared in the 2005 movie Kiss Kiss Bang Bang, which starred his father.
AP and Fairfax Media