Marijuana is used more than any other recreational drug, with recent trends toward greater social and legal acceptance in some regions. Concerns remain, however, about a possible causal relationship suggested in scientific studies between marijuana use and decline in IQ, particularly among adolescent users. Other analyses have found no or inconclusive evidence for a link between IQ decline and marijuana use. However, most prior research, regardless of conclusion, has suffered from small participant cohorts, absence of pre-use data, potential influence of confounding effects, or some combination of these factors.
READ MORE: http://news.meta.com/2016/01/18/twinsstudy/