Review
The hardening hypothesis: Is the ability to quit decreasing due to increasing nicotine dependence? A review and commentary

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Abstract

The “hardening hypothesis” states tobacco control activities have mostly influenced those smokers who found it easier to quit and, thus, remaining smokers are those who are less likely to stop smoking. This paper first describes a conceptual model for hardening. Then the paper describes important methodological distinctions (quit attempts vs. ability to remain abstinent as indicators, measures of hardening per se vs. measures of causes of hardening, and dependence measures that do vs. do not include cigarettes per day (cigs/day).) After this commentary, the paper reviews data from prior reviews and new searches for studies on one type of hardening: the decreasing ability to quit due to increasing nicotine dependence. Overall, all four studies of the general population of smokers found no evidence of decreased ability to quit; however, both secondary analyses of treatment-seeking smokers found quit rates were decreasing over time. Cigs/day and time-to-first cigarette measures of dependence did not increase over time; however, two studies found that DSM-defined dependence appeared to be increasing over time. Although these data suggest hardening may be occurring in treatment seekers but not in the general population of smokers, this conclusion may be premature given the small number of data sets and indirect measures of quit success and dependence in the data sets. Future studies should include questions about quit attempts, ability to abstain, treatment use, and multi-item dependence measures.

Introduction

Tobacco control has decreased the prevalence of smoking in the U.S.; however, this success has varied across populations (U.S. Dept Health and Human Services, 1989). For example, current smokers are more likely to have less education and income than prior smokers (MMWR, 2009). One hypothesis to explain such changes is that tobacco control has prompted the “easy-to-quit” smokers to quit and, thus, remaining smokers are those more resistant to stopping smoking. In 2003, two reviews of this “hardening” hypothesis concluded that (a) logically, one would expect hardening to occur; but (b) there is little empirical evidence of hardening (National Cancer Institute, 2003, Warner and Burns, 2003).

The purpose of the current article is to update these reviews. Because studies sometimes confuse measures of hardening with measures of the causes of hardening, and sometimes fail to distinguish quit attempts from successful abstinence after a quit attempt, the current article first presents a conceptualization of smoking cessation to facilitate discussion of hardening. Next, because definitions of hardening have become so idiosyncratic that they are substantially interfering with understanding the concept (Costa et al., 2010), the article suggests one definition (i.e., decreased ability to stop due to increased nicotine dependence) that is likely to be important to examine. Then, because the last review on hardening was 8 years ago, the paper presents more recent empirical tests of hardening. Finally, the paper suggests methodological assets to include in future studies.

Section snippets

A conceptualization of smoking cessation relevant to hardening

A change in smoking prevalence among adults can occur by a change in (a) the number of young smokers who become adult smokers, (b) the number of smokers who die, (c) the number of quit attempts, and (d) the ability to remain abstinent after a given quit attempt. Prior studies of hardening have focused on one or both of the latter two. In these studies, a quit attempt is typically defined as an attempt to stop smoking in the last year and often requires ≥24 h of abstinence (Hughes and Callas, 2010

Definitions of hardening

Hardening has been defined in many different ways (Costa et al., 2010, Hughes and Brandon, 2003). Some focus on the outcome; i.e., decreased quit attempts, decreased ability to abstain, or successful cessation. Others focus on possible causes; i.e., increased nicotine dependence, low motivation, low socioeconomic status and psychiatric co-morbidity. And others require both changes in outcomes and causes. This is problematic for several reasons. First, when definitions include several constructs

Populations in which hardening might be occurring

Hardening has often been thought of as a concept that should apply to the entire population of smokers; however, the concept has also been examined within subsets of smokers. The most commonly investigated subset has been treatment seekers. This distinction is important, because it is feasible that hardening could be occurring among treatment seekers but not in the population-at-large.

There are several other subgroups in which hardening might be occurring. One way to identify such subgroups is

Search for new studies

To locate possible new data on changes in the ability to remain abstinent over time due to nicotine dependence in July, 2010, the author searched PubMed, PsychInfo, Cochrane Library, clinicaltrials.gov, CRISP, CDC Tobacco Information and Prevention Database, and EMBASE for articles whose title, abstract or keyword included “hard* AND (smok* OR tobacco OR cigar* OR nicotin*).” The author's own files were searched. Any leads found in the reference lists of the articles obtained were pursued.

Results covered in prior two reviews

The prior two reviews (Warner and Burns, 2003, National Cancer Institute, 2003) cited two population-based surveys that examined the ability to remain abstinent. The U.S. Tobacco Use Supplement to the Current Population Survey (TUS-CPS) (i.e., data from the U.S. census) provided a description of the proportion of smokers who were able to remain abstinent for 3+ months among smokers who tried to quit one or more times in the last year (National Cancer Institute, 2003, Warner and Burns, 2003).

Search for studies

To locate studies focusing on dependence, the author noted those articles in the above searches that had the terms “dependen*”’ and “addict*” in the abstract. Among the 87 articles, 22 seemed appropriate but only 5 were included.

Changes in nicotine dependence in population-based samples

Few surveys in the search used formal measures of nicotine dependence such as the Fagerstom Test for Nicotine Dependence (FTND) or the Diagnostic and Statistical Manual (DSM) (Piper et al., 2006). The most common proxy measure for dependence was cigs/day. In the prior

Overview

This paper suggests studies on hardening should (a) distinguish between whether hardening influences quit attempts or ability to remain abstinent, (b) distinguish between measures of hardening per se (e.g., ability to remain abstinent) vs. measures of presumed causes of hardening (e.g., dependence), (c) examine the ability to remain abstinent due to dependence, and (d) examine hardening in both treatment seekers and the general population of smokers. The review of existing data concludes

Role of funding source

The writing of this review was funded by Senior Scientist Award DA-00490 from the National Institute on Drug Abuse. The sponsor had no role in the writing of this report or the decision to submit the report.

Conflict of interest

Dr. Hughes is currently employed by The University of Vermont and Fletcher Allen Health Care. Since 1/1/2008, he has received research grants from the National Institute on Health and Pfizer; the later develops and sells smoking cessation medications. During this time, he has accepted honoraria or consulting fees from several non-profit and for-profit organizations and companies that develop, sell or promote smoking cessation products or services or educate/advocate about smoking cessation:

Acknowledgements

The author thanks David Burns and Kenneth Warner for comments on an early draft of this paper.

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