What is the evidence for hardening in the cigarette smoking population? Trends in nicotine dependence in the U.S., 2002-2012

We did not find evidence of hardening at the population level for smokers in the U.S., 2002-2012. However, there is evidence of hardening when considering drive and continuity-related nicotine dependence among women and low-income smokers, suggesting these sub-groups are experiencing greater severity of craving, smoking to relieve negative affect, and regularity of smoking despite reduced consumption.