[kahnpro]

Or is it that people who have schizophrenia often belong to social classes that are more likely to use drugs?

If poor people are more likely to use drugs, and poor people also have a higher incidence of schizophrenia, you could arrive at this correlation even if marijuana and psychosis are entirely unrelated.

[mistersquid]

Marijuana use actually may be more strongly correlated to higher socioeconomic status than lower.

A quick Google search for "drug use by socioeconomic status" turns up a 2012 article titled "Socioeconomic Status and Substance Use Among Young Adults: A Comparison Across Constructs and Drugs" [0] which concludes, in part

  > Findings based on three indicators of family background
  > SES—income, wealth, and parental education—converged in
  > describing unique patterns for smoking and for alcohol and
  > marijuana use among young adults, although functional
  > relationships across SES measures varied. Young adults with
  > the highest family background SES were most prone to alcohol
  > and marijuana use.

[0] https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3410945/

EDIT: Change lede.

[Broken_Hippo]

Poor folks aren't more likely to use drugs: It seems that drug abuse is actually pretty common through society, but with poor folks using the least - mostly because drugs cost money.

Poor people are more likely to get caught for using drugs, however, simply because of living spaces. You and a couple friends wanna get high? Low income, you might be in a car, on the corner, or in an alley. Higher income, you might be in your basement or pretty private backyard.