Decriminalization should reduce crime by definition I think as fewer things are classified as crimes.
edit: my bad - "Decriminalization or decriminalisation is the lessening or termination of criminal penalties in relation to certain acts" so these things can still be classified as crimes
edit: my bad - "Decriminalization or decriminalisation is the lessening or termination of criminal penalties in relation to certain acts" so these things can still be classified as crimes