yes you are right about the population growth, I thought this too - but think of it like a school year or something like that (i.e. there are, in most industrialised countries, pretty much the same number of people going into a year of school as the year before - we have a rectangular (as opposed to pyramidal) population tree. Most people experiment with drugs in the same age groups, and In australia over the last 15 years heroin use has been falling (although slight bump last year or 2) - thus the 'cohort' or number of people trying heroin is not increasing, certainly not at the rate of population growth. cheers!