Is health literacy an example of construct proliferation? A conceptual and empirical evaluation of its redundancy with general cognitive ability
Introduction
Health literacy has become a topic of significant interest among health and medical researchers during the past two decades, particularly in regard to its potential explanatory role in health disparities. For example, low health literacy has been shown to be associated with a variety of health outcomes including increased risk of chronic health problems and decreased utilization of health care services (Berkman et al., 2004). Findings such as these have compelled health researchers to consider the elucidatory role of cognitive factors with respect to individual and group differences in health outcomes. At the same time, researchers in the science of mental abilities have amassed evidence that individual differences in basic cognitive abilities, in particular g (the general mental ability construct), are associated with a variety of health behaviors and health outcomes. For example, it has been shown that measures of cognitive abilities predict health behaviors such as amount of physical activity, eating fruits and vegetables, taking vitamins, and smoking (e.g., Anstey et al., 2009, Batty et al., 2007).
Despite the apparent similarity between these streams of research, there has been little effort to understand how the health literacy construct fits within the broader nomological network of intelligence. In fact, it would appear that evidence demonstrating the influence of cognitive abilities on health outcomes is still relatively unknown health psychology, medicine and associated fields. To date, only a few studies have investigated how cognitive abilities and health literacy are associated (e.g., Mõttus et al., 2013, Murray et al., 2011, Wolf et al., 2012). For example, Mottus et al recently showed that health literacy tests have somewhat limited incremental validity after accounting for cognitive ability and education. As such, it remains unclear to what extent health literacy measures are assessing construct variance that is distinct from basic cognitive abilities.
We believe this is a potentially important oversight as it creates the conditions for construct redundancy and construct proliferation. Construct proliferation and redundancy has been noted as a major problem in psychology (e.g., Le et al., 2010, Morrow, 1983, Schwab, 1980) and can be viewed as a major failure to adhere to the canon of parsimony in science. As these authors have noted, “a science that ignores the mandate for parsimony cannot advance its knowledge base and achieve cumulative knowledge.” (Le et al., 2010, p. 112). We believe a clear understanding of the role of cognitive differences in health outcomes will emerge more quickly with an accurate understanding of the construct space being assessed by purported measures of health literacy. As such, our purpose is to evaluate the conceptual and empirical uniqueness of the health literacy construct compared to known cognitive abilities.
Within the larger process of construct conceptualization and validation, it is imperative to demonstrate the uniqueness of a construct by either its complete or partial independence from other comparable constructs (Cronbach and Meehl, 1955, Messick, 1989). The first step of that process requires the construct definition and nomological network of the target construct to be conceptually distinct from existing validated constructs. To avoid construct proliferation and redundancy, it is incumbent upon the newer proposed construct to distinguish itself from known constructs. In cases of where concepts compete for the same construct space, the parsimonious model is accepted as the superior model.
To begin, we can compare their definitions. Cognitive abilities (or “mental abilities”) in general are defined as the sources of variance in performance on tasks requiring one to mentally process, comprehend, and manipulate information (Carroll, 1993, Reeve and Bonaccio, 2011a). While there are several specific abilities (e.g., quantitative reasoning; visual–spatial perception; cognitive speed), the general cognitive ability underlying all of these, or ‘g’ as it is known, has been defined as the factor reflecting individual differences in the ability to educe relations and correlations since it was first formally proposed by Jensen, 1998, Reeve and Bonaccio, 2011a, Spearman, 1904. As Gottfredson (2009) eloquently interpreted, this means “the generalized capacity to learn, reason and solve problems in essentially any context”. Operationally, this means general ability would manifest behaviorally as the ability to obtain and understand information (i.e., to learn), process information (i.e., reason), and use information to make appropriate decisions (i.e., solve problems in context). Although the definition of health literacy used by researchers has evolved in the past fifteen years, the most commonly cited definition of health literacy is that adopted by the Institute of Medicine (Nielson-Bohlman, Panzer, & Kindig, 2004), the Department of Health and Human Services (Healthy People, 2010) and the World Health Organization (WHO) (1998) which states that health literacy is “the ability to obtain, process, and understand basic information and services need to make appropriate health decisions.” Given the accepted definitions, it would appear that health literacy is essentially a restatement of the extant definition of ‘g,’ as it would be manifest in a specific context. That is, it is well established that general cognitive ability (‘g’) is indifferent to the indicator (Jensen, 1998, Spearman, 1927). This principle states that it is not the surface level features of items or situations that determine how well it measures ‘g’, but rather it is the underlying cognitive process. Any situation, regardless of context, that requires the eduction of relations and correlates and the application of knowledge to novel problem solving situations will measure ‘g’. Thus, it appears that the primary conceptual difference is not in the cognitive processes per se, but in whether a model positing a general information processing capacity is more or less parsimonious than a componential model comprised of numerous domain-specific faculties.
In this respect, it has been predicted and confirmed that individual differences in health literacy scores are associated with health specific outcomes such as lower depression scores (Gazmararian, Baker, Parker, & Blazer, 2000), measures of morbidity, general health status, and use of health resources (DeWalt, Berkman, Sheridan, Lohr, & Pignone, 2004), mortality risk (Baker et al., 2007). However, in keeping with the definition g, Gottfredson (2004) posited that individual differences in g would manifest as the ability to comprehend, interpret and apply information in a health care context just as it has been shown to do so in educational, occupational, and social contexts (Kuncel, Hezlett, & Ones, 2004). This prediction has also been well supported. For example, evidence shows that individual differences in g are associated with lower depression scores, better general health, and significantly lower odds for stroke, congestive heart failure, chronic lung disease, heart problems, and hypertension later in life (Der, Batty, & Deary, 2009), reduced risk of mortality (Batty et al., 2008, Deary et al., 2003), reduced likelihood of smoking (Anstey et al., 2009, Reeve and Basalik, 2010), and higher diet quality and increased physical activity (Anstey et al., 2009, Batty et al., 2007) and other indicators of health (e.g., Mõttus et al., 2013). Thus again, this suggests potential for construct redundancy.
Thus, the conceptual question centers on the strength of the evidential basis for the most parsimonious theory. In this case, the empirical reality and the importance of the g construct have been well documented for more than 100 years. Going back to the early 1900s, a broad array of psychometric, biological, and behavioral genetic evidence (e.g., see Carroll, 1993, Jensen, 1998) has given rise to a broad consensus among experts that cognitive abilities are real psychological constructs (Gottfredson, 1997a, Reeve and Charles, 2008, Snyderman and Rothman, 1987), that they have a significant and meaningful influence on important real-world outcomes (e.g. Gottfredson, 1997b, Gottfredson, 2004, Jensen, 1998, Kuncel et al., 2004, Schmidt and Hunter, 1998), that they have a basis in human biology and physiology (Deary, 2009, Gottfredson, 1997b [special issues of Intelligence]; Lubinski, 2004 [special section of the Journal of Personality and Social Psychology]) and is consistent with predictions from evolutionary psychology (Kanazawa, 2004, Kanazawa, 2010). In contrast, the term “health literacy” did not appear until 1974 and a formal treatment of health literacy as a construct did not emerge until the late 1990s (Mancuso, 2009). Additionally, the proposal of a second domain-specific construct to replace a domain-independent construct is less parsimonious, and lacks a basis in the biology or physiology of the central nervous system. Moreover, it is not possible for a domain-specific adaptation of the brain to have evolved within the last century (arguably the time frame the domain in question has been in existence). Conceptually, it is difficult to see where or how health literacy might carve out a unique space within the existing network known as intelligence.
Recent research from IO psychology demonstrates that contextualized ability measures can yield incremental predictive validity above traditional ability measures despite denoting the same construct as the traditional measure (e.g., Klingner & Schuler, 2004). This suggests that contextualized ability measures may assess some unique variance in addition to the basic cognitive abilities that are relevant to context specific criteria. Thus, it remains possible that even if health literacy as a construct is not unique, the measures used do assess some unique cognitive skill that is distinct from general cognitive abilities. As such, we conducted an empirical examination of the convergent and discriminant validity of the three health literacy measures to better understand their empirical distinctiveness from basic abilities.
Section snippets
Participants
A total of 169 adults participated. Everyone enrolled in an introductory psychology course at a mid-size urban (largely commuter) university located in the southeastern U.S. were recruited through the university's undergraduate student research pool. Two participants did not complete the demographic background information. The remaining 167 participants (117 females and 50 male) were between the ages of 18 and 53 (M = 21.31, SD = 5.61), and mostly self-identified as white (62% white, 25.3% as
Descriptive analyses
Descriptive statistics and uncorrected zero-order correlations among the ability and health literacy measures are shown in Table 1. Compared to the published norms for a general population of college students, the current sample scored, on average, at the 30th percentile on both the verbal comprehension and reasoning tests, and at the 15th percentile on both numerical tests. Though low, these scores are consistent with the SAT scores of the local population of psychology majors typically
Discussion
The purpose of this study was to evaluate the uniqueness of the health literacy construct and the validity of health literacy measures relative to cognitive abilities. Based on the totality of the evidence presented here, it is difficult to conclude that “health literacy” reflects a unique construct or that so-called health-literacy tests are psychometrically adequate measures. This conclusion is based on several lines of evidence. First, our conceptual analysis of the definition of health
Conclusion
The current topic provides a critical demonstration of the need for all areas of social science to better heed the principles of measurement. As we noted earlier, posting new constructs similar to existing ones (the “old wine in new wineskins” phenomenon) can be viewed as a major failure to adhere to the canon of parsimony in science. If the mandate of parsimony were ignored, it would seem unlikely that we will achieve the broad scientific goal of uncovering the fundamental causes that underlie
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