Smokers Increasingly Motivated and Able to Quit as Smoking Prevalence Falls: Umbrella and Systematic Review of Evidence Relevant to the “Hardening Hypothesis,” Considering Transcendence of Manufactured Doubt

Abstract

Introduction The “hardening hypothesis” proposes that as the prevalence of smoking in a population declines, there will be a “hardening” of the remaining smoker population. This review examines the evidence regarding smokers’ motivation, dependence and quitting behaviour as smoking prevalence declines, to assess whether population “hardening” (decreasing propensity to quit) or “softening” (the converse) is occurring. Methods MEDLINE, PsychINFO, Scopus, Web of Science and Cochrane Library were searched to July 2019, using terms related to smoking and hardening, for reviews and large, population-based repeat cross-sectional studies. There were additional searches of reference lists and citations of key research articles. Two reviewers screened half the titles and abstracts each, and two reviewers screened full texts independently using tested criteria. Four reviewers independently and systematically extracted data from eligible publications, with one reviewer per study, checked by another reviewer. Results Of 265 titles identified, three reviews and ten repeat cross-sectional studies were included. Reviews concluded that hardening has not occurred among the general smoking population over time. Among repeated cross-sectional studies, five examined motivation, nine examined dependence, five examined hardcore smoking, and two examined quit outcomes. All but one study found a lack of hardening. Most found softening within the smoking population, consistent across hardening indicators, definitions, countries (and tobacco control environments) and time periods examined. Conclusions Tobacco control reduces smoking prevalence and fosters a smoking population more amenable to evidence-based interventions. Based on the weight of the available evidence, the “hardening hypothesis” should be rejected and the reality of softening accepted. Implications This umbrella review and systematic review provides a critical consideration of evidence from psychology and other fields regarding the “hardening hypothesis” – a persistent myth undermining tobacco control. It reaches the conclusion that the sum-total of the world-wide evidence indicates either “softening” of the smoking population, or a lack of hardening. Hence, tobacco control reduces smoking prevalence and fosters a smoking population more amenable to evidence-based interventions. The review indicates that the time has come to take active steps to combat the myth of hardening and to replace it with the reality of “softening”.

The Hardening Hypothesis: Further Testing Is Still Required

“…Adding to the continuing discussions, a recently-published umbrella and systematic review by Harris et al strongly opposed the existence of the hardening hypothesis, after conducting a wide-ranging qualitative synthesis of the evidence pertinent to various hardening constructs and indicators. 1 However, several key methodological issues may have ushered a deficient—or worse, unqualified—interpretation of the evidence base.…”

“…Although it is understandable that systematic reviews do not extract and synthesize all outcomes of studies included around a certain topic, the method is usually guided by a predefined order of measures. However, in the review by Harris et al 1 , we were surprised that the authors did not include prioritization of outcome measures in their methodology, and included instances where approximate measures were presented over more definitive measures. For example, the review used the study by Edwards et al 2 to claim that because smokers’ attitudes to tobacco control measures and goals have softened, their motivation to quit has increased.…”

“…For example, the review used the study by Edwards et al 2 to claim that because smokers’ attitudes to tobacco control measures and goals have softened, their motivation to quit has increased. However, Edwards et al 2 also presented results of a more direct measure of motivation to quit, reporting an increase in the proportion of daily smokers who made no attempts to quit lasting more than 24 hours in the past 12 months (although adjusted models found no significant increase); this measure was not discussed at all by Harris et al 1 …”

“…Furthermore, Harris et al 1 failed to account for alternative causes for the observed softening of smokers by not considering confounders present in the studies. Indeed, in line with the hardening hypothesis that Harris et al 1 rejects, a possible explanation for the observed softening could have been an increase in the uptake of harm reduction tools among hardened smokers.…”

“…Furthermore, Harris et al 1 failed to account for alternative causes for the observed softening of smokers by not considering confounders present in the studies. Indeed, in line with the hardening hypothesis that Harris et al 1 rejects, a possible explanation for the observed softening could have been an increase in the uptake of harm reduction tools among hardened smokers. The authors acknowledge that only “two studies adjusted for concomitant nicotine administration in the form of snus and nicotine replacement therapy.” Therefore, many of the included studies may have been confounded by the use of harm reduction tools; notably, the need to consider such treatment is not novel and was even discussed in the included Hughes 3 review.…”