Alcohol consumption, life context, and coping predict mortality among late-middle-aged drinkers and former drinkers

Association Between Daily Alcohol Intake and Risk of All-Cause Mortality: A Systematic Review and Meta-analyses

Importance

A previous meta-analysis of the association between alcohol use and all-cause mortality found no statistically significant reductions in mortality risk at low levels of consumption compared with lifetime nondrinkers. However, the risk estimates may have been affected by the number and quality of studies then available, especially those for women and younger cohorts.

Objective

To investigate the association between alcohol use and all-cause mortality, and how sources of bias may change results.

Data Sources

A systematic search of PubMed and Web of Science was performed to identify studies published between January 1980 and July 2021.

Study Selection

Cohort studies were identified by systematic review to facilitate comparisons of studies with and without some degree of controls for biases affecting distinctions between abstainers and drinkers. The review identified 107 studies of alcohol use and all-cause mortality published from 1980 to July 2021.

Data Extraction and Synthesis

Mixed linear regression models were used to model relative risks, first pooled for all studies and then stratified by cohort median age (<56 vs ≥56 years) and sex (male vs female). Data were analyzed from September 2021 to August 2022.

Main Outcomes and Measures

Relative risk estimates for the association between mean daily alcohol intake and all-cause mortality.

Results

There were 724 risk estimates of all-cause mortality due to alcohol intake from the 107 cohort studies (4 838 825 participants and 425 564 deaths available) for the analysis. In models adjusting for potential confounding effects of sampling variation, former drinker bias, and other prespecified study-level quality criteria, the meta-analysis of all 107 included studies found no significantly reduced risk of all-cause mortality among occasional (>0 to <1.3 g of ethanol per day; relative risk [RR], 0.96; 95% CI, 0.86-1.06; P = .41) or low-volume drinkers (1.3-24.0 g per day; RR, 0.93; P = .07) compared with lifetime nondrinkers. In the fully adjusted model, there was a nonsignificantly increased risk of all-cause mortality among drinkers who drank 25 to 44 g per day (RR, 1.05; P = .28) and significantly increased risk for drinkers who drank 45 to 64 and 65 or more grams per day (RR, 1.19 and 1.35; P < .001). There were significantly larger risks of mortality among female drinkers compared with female lifetime nondrinkers (RR, 1.22; P = .03).

Conclusions and Relevance

In this updated systematic review and meta-analysis, daily low or moderate alcohol intake was not significantly associated with all-cause mortality risk, while increased risk was evident at higher consumption levels, starting at lower levels for women than men.

Residual Effects: Young Adult Diagnostic Drinking Predicts Late-Life Health Outcomes

OBJECTIVE

This study examined the residual effects of young adult diagnostic drinking on health outcomes four decades later in late life. Results were differentiated by drinking status during midlife.

METHOD

A subsample of Vietnam Era Twin Registry members, all of whom had a lifetime diagnosis of alcohol dependence, was grouped according to life span drinking patterns as assessed by the Lifetime Drinking History interview in 2001. Those drinking at diagnostic levels (endorsing three or more alcohol dependence symptoms) before age 30 were then grouped based on their midlife drinking status (i.e., drinking at diagnostic levels vs. at minimal [nonsymptomatic] levels throughout midlife). Linear (or logistic) regression models were used to examine the association between life span drinking patterns and health outcomes in late life (about age 64).

RESULTS

Those who drank at diagnostic levels in young adulthood and in midlife exhibited significant health liabilities on every late-life health measure; those who drank at diagnostic levels for 5 or more years in young adulthood but drank only at minimal levels or not at all in midlife still exhibited similar liabilities on most late-life health measures. Only those individuals who drank diagnostically for less than 5 years in young adulthood displayed normal levels of late-life health.

CONCLUSIONS

This study identified residual effects resulting from persistent young adult diagnostic drinking (5 or more years) that resulted in negative health outcomes in late life even after decades of remission. There is a distal but surprisingly strong association between persistent early life diagnostic drinking and late-life morbidity.

A Longitudinal Study of Religiosity and Mortality Risk

The relation of adult religiosity to longevity was studied in 993 participants from Terman's 70-year Life-Cycle Study. Key social and behavioral variables of physical health, psychological well-being, socioeconomic status, social support, and health behaviors were also considered. Results indicate that women who viewed themselves as more religious in adulthood (approximately age 40) had a lower risk for premature mortality than those who were less religiously inclined. These women had healthier behaviors, more positive feelings about their futures, and reported being somewhat happier than their less religiously inclined peers. In this bright, middle-class, 20th century sample, religiosity among women seems to be part of a generally healthy lifestyle, but not necessarily a direct cause of it.

Alcohol and All-Cause Mortality: A Pooled Analysis

Reviewing 59 studies of the risk curve for alcohol consumption level and all-cause mortality, a general J-shaped curve is confirmed, particularly in cohorts with experience starting at middle age (rather than in youth or old age). The divergence in most studies that are exceptions to this rule can be plausibly explained. A pooled analysis of studies with cohorts age 45 and above at entry shows the most beneficial effect for women in a drinking category with a midpoint of one drink every second day, and for men in the drinking category with a midpoint of one and a half drinks per day though most of the benefit can be obtained by men, too, in the category with a midpoint of one-half drink per day. For men under 45, there was an almost linear increase in mortality risk with increasing consumption; data are lacking for a similar analysis for women. Directions are suggested for development in future studies, including the use of data on patterns of drinking.

Episodic heavy drinking and 20-year total mortality among late-life moderate drinkers

BACKGROUND

Analyses of moderate drinking have focused overwhelmingly on average consumption, which masks diverse underlying drinking patterns. This study examined the association between episodic heavy drinking and total mortality among moderate-drinking older adults.

METHODS

At baseline, the sample was comprised of 446 adults aged 55 to 65; 74 moderate drinkers who engaged in episodic heavy drinking and 372 regular moderate drinkers. The database at baseline also included a broad set of sociodemographic, behavioral, and health status covariates. Death across a 20-year follow-up period was confirmed primarily by death certificate.

RESULTS

In multiple logistic regression analyses, after adjusting for all covariates, as well as overall alcohol consumption, moderate drinkers who engaged in episodic heavy drinking had more than 2 times higher odds of 20-year mortality in comparison with regular moderate drinkers.

CONCLUSIONS

Among older moderate drinkers, those who engage in episodic heavy drinking show significantly increased total mortality risk compared to regular moderate drinkers. Episodic heavy drinking-even when average consumption remains moderate-is a significant public health concern.

Wine consumption and 20-year mortality among late-life moderate drinkers

OBJECTIVE

This study examined level of wine consumption and total mortality among 802 older adults ages 55-65 at baseline, controlling for key sociodemographic, behavioral, and health status factors. Despite a growing consensus that moderate alcohol consumption is associated with reduced total mortality, whether wine consumption provides an additional, unique protective effect is unresolved.

METHOD

Participants were categorized in three subsamples: abstainers, high-wine-consumption moderate drinkers, and low-wine-consumption moderate drinkers. Alcohol consumption, sociodemographic factors, health behavior, and health problems were assessed at baseline; total mortality was indexed across an ensuing 20-year period.

RESULTS

After adjusting for all covariates, both high-wine-consumption and low-wine-consumption moderate drinkers showed reduced mortality risks compared with abstainers. Further, compared with moderate drinkers for whom a high proportion of ethanol came from wine, those for whom a low proportion of ethanol came from wine were older, were more likely to be male, reported more health problems, were more likely to be tobacco smokers, scored lower on socioeconomic status, and (statistical trend) reported engaging in less physical activity. Controlling only for overall ethanol consumption, compared with moderate drinkers for whom a high proportion of ethanol came from wine, those for whom a low proportion of ethanol came from wine showed a substantially increased 20-year mortality risk of 85%. However, after controlling for all covariates, the initial mortality difference associated with wine consumption was no longer significant.

CONCLUSIONS

Among older adults who are moderate drinkers, the apparent unique effects of wine on longevity may be explained by confounding factors correlated with wine consumption.

Impulsivity is an independent predictor of 15-year mortality risk among individuals seeking help for alcohol-related problems

BACKGROUND

Although past research has found impulsivity to be a significant predictor of mortality, no studies have tested this association in samples of individuals with alcohol-related problems or examined moderation of this effect via socio-contextual processes. The current study addressed these issues in a mixed-gender sample of individuals seeking help for alcohol-related problems.

METHODS

Using Cox proportional hazard models, variables measured at baseline and Year 1 of a 16-year prospective study were used to predict the probability of death from Years 1 to 16 (i.e., 15-year mortality risk). There were 628 participants at baseline (47.1% women); 515 and 405 participated in the follow-up assessments at Years 1 and 16, respectively. Among Year 1 participants, 93 individuals were known to have died between Years 1 and 16.

RESULTS

After controlling for age, gender, and marital status, higher impulsivity at baseline was associated with an increased risk of mortality from Years 1 to 16; however, this association was accounted for by the severity of alcohol use at baseline. In contrast, higher impulsivity at Year 1 was associated with an increased risk of mortality from Years 1 to 16, and remained significant when accounting for the severity of alcohol use, as well as physical health problems, emotional discharge coping, and interpersonal stress and support at Year 1. In addition, the association between Year 1 impulsivity and 15-year mortality risk was moderated by interpersonal support at Year 1, such that individuals high on impulsivity had a lower mortality risk when peer/friend support was high than when it was low.

CONCLUSIONS

The findings highlight impulsivity as a robust and independent predictor of mortality and suggest the need to consider interactions between personality traits and socio-contextual processes in the prediction of health-related outcomes for individuals with alcohol use disorders.

Severity of alcohol-related problems and mortality: results from a 20-year prospective epidemiological community study

There is evidence that high alcohol use is associated with an increase in mortality. Little is known about long-term effects of problematic alcohol consumption in non-clinical (community) populations. The aim of our study was to obtain data on this and related issues in a representative rural community sample assessed longitudinally over a period of 20 years. Assessments focused on a baseline survey from 1980 to 1984 and 20-year follow-up from 2001 to 2004. Based on expert interviews and standardized self-rating scales (e.g. MALT; Munich Alcoholism Test), the following three groups were defined (a) severe alcohol problems, (b) moderate alcohol problems, and (c) no alcohol problems. Mortality and hazard rates were analyzed with logistic and Cox regression adjusted for several health risk factors. From an original community sample of 1,465 individuals, 448 were deceased at 20-year follow-up. Participation rates were high. Baseline prevalence according to the MALT was 1.6% for severe alcohol problems and 4.0% for moderate alcohol problems. Over the 20-year time span, individuals with severe alcohol problems had a significantly elevated risk for dying earlier than the group with no alcohol problems (2.4 times higher). Mortality for those with moderate alcohol problems at baseline had a non-significantly elevated 20-year mortality risk (1.5 times higher) compared to those with no alcohol problems. Cox survival analyses corroborate these findings from multiple sequential logistic regression analyses. In discussing the mortality risk of persons with alcohol problems, the severity of the alcohol problems must be taken into account.

Coping and Control Processes: Do They Contribute to Individual Differences in Health in Older Adults?

This paper reviews the literature on the relation of coping and control processes to health outcomes in late adulthood and presents new data on relations between coping and control processes and health for 295 World War II veterans. The results for the veterans showed that health was positively associated with cognitive coping, and negatively associated with behavioural coping and avoidance. No association was found between perceived locus of control and health. These findings, together with those in the literature, were discussed in terms of their implications for future research on the role of coping and control in health maintenance and their significance for people working with older persons.

Social relationships and mortality risk: a meta-analytic review

BACKGROUND

The quality and quantity of individuals' social relationships has been linked not only to mental health but also to both morbidity and mortality.

OBJECTIVES

This meta-analytic review was conducted to determine the extent to which social relationships influence risk for mortality, which aspects of social relationships are most highly predictive, and which factors may moderate the risk.

DATA EXTRACTION

Data were extracted on several participant characteristics, including cause of mortality, initial health status, and pre-existing health conditions, as well as on study characteristics, including length of follow-up and type of assessment of social relationships.

RESULTS

Across 148 studies (308,849 participants), the random effects weighted average effect size was OR = 1.50 (95% CI 1.42 to 1.59), indicating a 50% increased likelihood of survival for participants with stronger social relationships. This finding remained consistent across age, sex, initial health status, cause of death, and follow-up period. Significant differences were found across the type of social measurement evaluated (p<0.001); the association was strongest for complex measures of social integration (OR = 1.91; 95% CI 1.63 to 2.23) and lowest for binary indicators of residential status (living alone versus with others) (OR = 1.19; 95% CI 0.99 to 1.44).

CONCLUSIONS

The influence of social relationships on risk for mortality is comparable with well-established risk factors for mortality. Please see later in the article for the Editors' Summary.

Medical Conditions of Hazardous Drinkers and Drug Users in Primary Care Clinics in Cape Town, South Africa

Research has identified a wide range of health conditions related to alcohol and drug use in studies conducted primarily in developed countries and in populations with severe alcohol and drug problems. Little is known about medical conditions in those with less severe alcohol and drug use in developing countries. We used WHO AUDIT and ASSIST screeners to identify hazardous drinking or drug use in public health clinics in Cape Town, South Africa, and included questions about doctor-diagnosed medical conditions. Using logistic regression we examined the relationship of medical conditions to hazardous alcohol, drug and tobacco use. Those with hazardous substance use had higher prevalence of many health conditions including tuberculosis. Hepatitis B, migraine, chronic bronchitis, and liver cirrhosis. Optimal treatment for some medical conditions may include treatment of underlying hazardous substance use, particularly use of drugs other than alcohol. In these populations, access to substance use treatment is limited and even brief interventions or advice may be useful.

Is the cardioprotective effect of alcohol real?

A large number of investigations in experimental, clinical, and epidemiological settings have given support to the idea that consumption of moderate amounts of alcoholic beverages, particularly wine, protects against coronary heart disease (CHD). Biological effects of other components of wine in human beings, however, have been hardly demonstrated, and alcohol itself has several potential adverse effects on the cardiovascular system. Not all epidemiological surveys have found protection from alcoholic beverages and in African-Americans, alcohol consumption was a risk factor for the incidence of CHD. The possibility that the lower risk of drinkers of moderate amounts of wine or other beverages is secondary to a health cohort effect in whites is not negligible, and could be discarded only in a clinical trial. In view of the potential risks of alcohol, a more cautious view about the beneficial effects of alcoholic beverages is warranted.

Effects of chronic swim stress on EtOH-related behaviors in C57BL/6J, DBA/2J and BALB/cByJ mice

There is a strong clinical relationship between stress and stress-related disorders and the incidence of alcohol abuse and alcoholism, and this relationship appears to be partly genetic in origin. There are marked strain differences in ethanol (EtOH)-related behaviors and reactivity to stress, but little investigation of the interaction between the two. The present study assessed the effects of chronic exposure to swim stress on EtOH-related behavior in three common inbred strains of mice, C57BL/6J, DBA/2J and BALB/cByJ. After establishing baseline (10%) EtOH self-administration in a two-bottle free choice test, mice were exposed to daily swim stress for 14 consecutive days and EtOH consumption was measured as a percent of baseline both during stress and for 10 days afterwards. A separate experiment examined the effects of 14 days of swim stress on sensitivity to the sedative/hypnotic effects of an acute injection of 4g/kg EtOH. Results showed that stress produced a significant decrease in EtOH consumption, relative to pre-stress baseline, in DBA/2J and BALB/cByJ, but not C57BL/6J mice. By contrast, stress increased sensitivity to the sedative/hypnotic effects of EtOH in all three strains. These findings demonstrate that chronic swim stress produces reductions in EtOH self-administration in a strain-dependent manner, and that these effects may be restricted to strains with a pre-existing aversion to EtOH. Present data also demonstrates a dissociation between effects of this stressor on EtOH self-administration and sensitivity to EtOH's sedative/hypnotic effects. In conclusion, strain differences, that are likely in large part genetic in nature, modify the effects of this stressor on EtOH's effects in a behavior-specific manner.

Chronic swim stress alters sensitivity to acute behavioral effects of ethanol in mice

Epidemiological data support a strong link between stress, stress-related disorders and risk for alcoholism. However, precisely how stress might impact sensitivity to the intoxicating effects of ethanol or the willingness to voluntary consume ethanol remains unclear. The present study assessed the effects of daily exposure to forced swim stress on subsequent sensitivity to the sedative/hypnotic, hypothermic, ataxic (measured using accelerating rotarod), and anxiolytic-like (measured using elevated plus-maze) effects of ethanol, and ethanol consumption and preference in a two-bottle choice paradigm, in male C57BL/6J mice. Stress effects on the sedative/hypnotic effects of the barbiturate pentobarbital were also tested. Results showed that chronic (fourteen days) but not acute (one or three days) swim stress significantly potentiated the sedative/hypnotic and hypothermic effects of 4 g/kg, but not 3 g/kg, ethanol. The sedative/hypnotic effects of pentobarbital were attenuated by chronic swim stress. Irrespective of chronicity, swim stress did not alter the ataxic or anxiolytic-like effects of ethanol, or alter ethanol self-administration either during or after stress. These data provide further evidence that stress alters the intoxicating effects of high doses of ethanol in a behaviorally selective manner.

Predictors of 16-Year Mortality Among Individuals Initiating Help-Seeking for an Alcoholic Use Disorder

BACKGROUND AND METHODS

We examined rates and predictors of mortality in individuals (47% women) who had just initiated help-seeking for their alcohol use disorders (AUDs) at the start of the study (n=628) and were followed for 16 years.

RESULTS

For both women and men, the observed-to-expected mortality ratio (1.4) was lower than rates found in samples of treated individuals with AUDs, suggesting that those initiating help-seeking careers have better chances of long-term survival. Of the individuals for whom cause of death was known, 68% died of alcohol-related causes. Men were more likely to die than were women. When gender was controlled, individuals who were older and unmarried and had more alcohol dependence symptoms at baseline were more likely to die over the 16-year period. When these baseline characteristics were controlled, better drinking outcomes at 1 year were associated with a lower likelihood of subsequent death. The combination of a shorter duration of inpatient/residential care and better drinking outcomes at 1 year was related to a lower probability of death, as was the combination of a longer duration of outpatient care or Alcoholics Anonymous attendance and better drinking outcomes at 1 year.

CONCLUSIONS

Efforts should be made to help providers identify individuals who are not responding positively to inpatient or residential treatment and intervene to motivate participation in continuing outpatient care and community 12-step self-help groups to reduce the likelihood of a chronic and fatal AUD course.

Stress generation, avoidance coping, and depressive symptoms: a 10-year model

This study examined (a) the role of avoidance coping in prospectively generating both chronic and acute life stressors and (b) the stress-generating role of avoidance coping as a prospective link to future depressive symptoms. Participants were 1,211 late-middle-aged individuals (500 women and 711 men) assessed 3 times over a 10-year period. As predicted, baseline avoidance coping was prospectively associated with both more chronic and more acute life stressors 4 years later. Furthermore, as predicted, these intervening life stressors linked baseline avoidance coping and depressive symptoms 10 years later, controlling for the influence of initial depressive symptoms. These findings broaden knowledge about the stress-generation process and elucidate a key mechanism through which avoidance coping is linked to depressive symptoms.

Hazardous drinkers and drug users in HMO primary care: prevalence, medical conditions, and costs

BACKGROUND

There exists substantial evidence that individuals with alcohol and drug disorders have heightened comorbidities and health care costs. However, little is known about the larger population of "hazardous" drinkers (those whose consumption increases their "risk of physical and psychological harm") and drug users.

METHODS

A sample of 1,419 patients from HMO primary care clinics was screened for hazardous drinking and drug use. Health plan databases were used to examine medical conditions and health care costs of hazardous drinkers and drug users in the year prior to screening, in comparison to 13,347 patients from the same clinics, excluding those screened.

RESULTS

We found a prevalence of 7.5% for hazardous drinking and 3.2% for drug use in primary care (10% had at least one of the two problems). Hazardous drinkers and drug users had heightened prevalences for eight medical conditions, including costly conditions such as injury and hypertension, and psychiatric conditions. Medical costs for the year examined were not higher, except for those who also had psychiatric conditions.

CONCLUSIONS

The prevalence of hazardous drinking and drug use was similar to hypertension and diabetes. Hazardous drinkers and drug users' heightened medical conditions, especially those related to alcohol and drug abuse, indicate that screening and brief intervention at this lower threshold of hazardous drinking and drug use will detect individuals with health risks sooner. Optimal treatment and prevention of some medical disorders may require identification and intervention of underlying hazardous alcohol or drug use.

Predictors of 5-year mortality following inpatient/residential group treatment for substance use disorders

This study examined the prevalence and predictors of 5-year mortality following treatment for substance use disorders. The predictors were assessed at baseline, at discharge, and at a 1-year follow-up for 3698 male veterans, and included demographic, substance use, medical, and psychological functioning, social support, and continuing care. The annual mortality rate was 2.38%, with an observed/expected ratio of 3.05. After accounting for significant demographic, substance use, psychological, and medical conditions, not having a spouse or partner at intake independently predicted 5-year mortality. After accounting for intake variables, more depression at discharge and more medical conditions, a diagnosis of HIV or AIDS, more ounces of ethanol on a maximum drinking day, and lack of a spouse or partner at the 1-year follow-up independently predicted 5-year mortality. Unexpectedly, good quality relationships were related to a higher mortality risk. Results can be used to increase at-risk patients' motivation for recovery.

Older adults' health and changes in late-life drinking patterns

This study focused on the prospective associations between older adults' health-related problems and their late-life alcohol consumption and drinking problems. A sample of 1,291 late-middle-aged community residents (55-65 years old at baseline) participated in a survey of health and alcohol consumption, and was followed one year, four years, and 10 years later. Health-related problems increased and alcohol consumption and drinking problems declined over the 10-year interval. Medical conditions, physical symptoms, medication use, and acute health events predicted a higher likelihood of abstinence and less frequent and lower alcohol consumption. However, overall health burden predicted more subsequent drinking problems, even after controlling for alcohol consumption and a history of heavy drinking and increased drinking in response to stressors. Among older adults, increased health problems predict reduced alcohol consumption but more drinking problems. Older adults with several health problems who consume more alcohol are at elevated risk for drinking problems and should be targeted for brief interventions to help them curtail their drinking.

Generalized Additive Models applied to analysis of the relation between amount and type of alcohol and all-cause mortality

The J-shaped relation between alcohol intake and mortality is well established, whereas the nadir of the curve is not determined. Due to non-linearity of the relation, categorical alcohol variables have been used to model the relation. In Generalized Additive Models (GAM) non-linear relations can be modelled without the disadvantages of categorization and without assumptions regarding the functional form. The aim of this study was to use GAM to evaluate the relation between alcohol intake, amount and type, and mortality. The relation was investigated using data from the Copenhagen City Heart Study (11,920 participants of whom 5552 died during 20 years follow-up). Using GAM, a smooth J-shaped relation between alcohol and mortality was found. However, if non-drinkers were categorized separately there was a positive association between alcohol and mortality even for low alcohol intake. For equal total alcohol intake, men and women drinking wine or spirits had lower mortality than beer drinkers. The nadir of the relation between alcohol and mortality was sensitive to the handling of non-drinkers. When non-drinkers were categorized separately we found no indication of a beneficial influence of low alcohol intake on mortality.

Personality, stress, and social support in cocaine relapse prediction

This study identified prospective psychosocial predictors of relapse status and drug abuse severity in male subjects in the first year after residential treatment for cocaine dependence. Personality, stress, and social support measures from an intake assessment, and stress and support measures reflecting status during the three-month period prior to the one in which relapse was identified were used as predictors. A number of hypotheses were confirmed. Detached personality and stress predicted both cocaine relapse and outcome drug abuse severity. Perceived social support quality and social network size predicted cocaine relapse. Implications for relapse prevention are presented.

Exploring psychological benefits associated with moderate alcohol use: a necessary corrective to assessments of drinking outcomes?

The aim of this paper is to identify positive psychological concomitants of moderate alcohol consumption. Current research and public-health perspectives on alcohol emphasize harms disproportionately relative to benefits. The major exception is research establishing beneficial effects of moderate drinking on cardiovascular health and overall mortality. In addition, much observational and experiential data suggest the widespread prevalence of positive drinking experiences. This paper is one of the first attempts since 1985 to codify such benefits in epidemiological terms. Methodological difficulties in accomplishing this include defining moderate drinking, controlling for confounding variables, and establishing causality. Nonetheless, evidence of psychological benefits has been found in experimental, observational, interview, self-report, correlational, and some prospective research. These positive findings are in the areas of subjective health, mood enhancement, stress reduction, sociability, social integration, mental health, long-term cognitive functioning, and work income/disability. Problem drinkers and alcoholics also seek mood and other benefits from alcohol, but are more likely to drink to counteract negative feelings and to support their egos than are social drinkers. It is as yet impossible to determine to what extent moderate alcohol consumption causes positive psychological outcomes and to what extent it is part of a complex pattern of mutually reinforcing variables.