The Relationship between Subjective Aging and Cognition in Elderly People: A Systematic Review

There is a growing body of evidence on the effects of subjective aging on health, well-being and quality of life. This review aims to synthesize findings about the link between subjective aging and cognition and cognitive decline. Furthermore, it provides an examination of variation sources such as subjective aging construct, cognitive domains, measures employed, age and moderator variables. A systematic search was performed in PubMed, PsychInfo and Web of Science, as well as grey literature searches in Google Scholar, OpenGrey, WorldCat and NDLTD, which resulted in 59 reports being included. Subjective aging is a relevant construct in the explanation and prediction of cognitive aging and cognitive decline in elderly adults. More positive views about own aging and self-perceptions of aging, as well as a younger subjective age, were consistently related to better cognition and lower risk of cognitive decline. However, there were differences due to subjective aging subdimensions and cognitive domains, as well as an effect of age. Additionally, there were concerns about the content validity of some measures employed, such as the Philadelphia Geriatric Center Morale Scale for subjective aging and the Mini Mental State Examination for global cognition. Further studies should employ longitudinal designs with a process-based approach to cognition and precise subjective aging measures.


Introduction
The progressive aging of the population is one of the challenges of contemporary societies, implying attainment of adequate levels of health and quality of life in the elderly population [1,2].While chronological age remains as a relevant predictor of aging trajectories, the progressive inclusion of social and psychosocial factors based on the active aging paradigm offers a multidimensional and more complete view [3,4].
Subjective aging is a relevant psychosocial variable in the prediction of health outcomes at elderly age.It is defined as the representations that arise from the subjective interpretation given to the personal process of aging and the attributions about one's own age.These representations encompass several elements related to the process of aging.Following the reconceptualization proposed by Barker et al. [5] of the common-sense model of regulation [6], elderly people can perceive different degrees of chronicity, control, consequences and the emotional impact of them.For instance, elder people might perceive that aging has positive and negative consequences and that they do or do not have personal control over them.Moreover, this impact can be viewed as something chronic or may arouse different emotional reactions.
Once these representations are formed, they might lead to different outcomes in health and quality of life.The stereotype embodiment theory [7] proposes three possible pathways.The psychological pathway explains how self-fulfilling prophecies about the nature, consequences, and control of the aging processes impact behavior.For instance, there is evidence of the link between negative representations and lower self-efficacy related to perceiving physical losses as out of control and having, as a consequence, a negative impact on physical health [8].Moreover, elderly people who engage in negative representations tend to use selection, optimization and compensation strategies to a lesser degree, specifically if they endorse a belief that physical losses are an inherent consequence of aging [9].The behavioral pathway explains how negative representations are related to several healthy behaviors that impact health, such as smoking [10], lack of adherence to pharmacological treatments [11], and physical activity levels [12].Lastly, the physiological pathway is related to autonomous nervous activation related to subjective aging.For example, elderly people with higher levels of negative representations show enhanced reactivity to stress, which increases the probability of a cardiac event [13].There is substantial evidence of the relationship between subjective aging and health outcomes in elderly people.Positive representations are linked to more years of life [14], higher functional health [15] and higher physical function [16], whereas negative representations are linked to a steeper decline in physical [17], and mental health [18].Meta-analytic approaches indicate a small but consistent effect of subjective aging on health and mortality [19] as well as on subjective well-being and depression [20].
This review focuses on exploring the relationship between subjective aging, cognitive functioning, and the probability of developing mild cognitive impairment or dementia.As the cognitive enrichment theory [21] proposes, elderly people are able to modify their cognitive trajectories through healthy behaviors and lifestyles.Even as cognitive efficacy is strongly associated with age because of biological and neurological factors, there is a substantial degree by which psychosocial factors, such as subjective representations of aging, can modify them.In theoretical terms, it can be argued that the content of this representation includes cognitive characteristics, such as 'distracted', 'forgetful' and 'wise' [22].These representations impact healthy behaviors, active lifestyles and selection, and optimization and compensation strategies, which, in turn, might lead to lower cognitive performance and a higher risk of developing neurodegenerative diseases [23,24].Moreover, negative representations are linked to variability in brain structures that support the main cognitive functions, such as lower hippocampal volume [25] and inferior grey matter volume in the inferior frontal gyrus and the superior temporal gyrus [26].There have been efforts to synthesize evidence regarding subjective age and cognition, finding a small but significant effect [20]; however, this systematic review aims to explore the complete umbrella concept of subjective aging as well as to address the potential sources of variation in this field.
First, there might be a disparity regarding the subjective aging construct addressed.There have been several conceptualizations employed, which are intertwined but do not reflect the same contents.Subjective age refers to the age someone feels he has, and it has usually been compared with chronological age [27].However, age identity is defined as the degree of correspondence between felt age and age associated with the peer group or social role [28].Self-perceptions of aging are representations that arise from the personal experience of aging [29], whereas attitudes towards own aging express self-directed cognitions, emotions and behaviors based on belonging to a certain age group [7].Moreover, the concept of awareness of aging is defined as the experiences that relate to changes associated with aging [30].Each of these constructs might impact differentially in the cognitive trajectories of elderly people.Theoretically, there might be differences between the pathways through which these constructs exert their effect.For instance, Westerhof & Wurm [31] proposed that age identity may contribute mainly to the psychological pathway since this construct is more closely related to dispositional variables such as optimism and self-esteem, whereas attitudes towards own aging might influence cognitive and behavioral pathways.This reasoning follows the current approaches to subjective aging, which considers the differences between highly aggregated measures of aging representations such as subjective aging and more specific and multidimensional approaches, as well as the specific processes involved in their relationships with health [32].Moreover, a matching effect [33] between the valence or content of the representation of aging and the health outcome has been proposed.For example, Spuling et al. [34] found that self-rated health is more strongly predicted by the specific physical losses dimension of self-perceptions of aging when compared with broader constructs such as subjective age and attitudes towards own aging.A similar effect was found by Sabatini et al. [35], showing that the awareness of age-related changes losses dimension predicts cognition but not general attitudes towards own aging.This effect may be explained by the inclusion of specific cognitive items in the losses dimension, whereas attitudes towards own aging reflect general representations.Finally, there is evidence that some subjective aging constructs are more consistent when predicting several cognitive domains, such as awareness of age-related changes when compared with attitudes towards own aging [36].
A second source of variation is the measures employed to assess subjective aging.Even if there is a wide spectrum of questionnaires, most of them have not been designed to measure these constructs [37], and some scales include items referring both to general beliefs about aging and to personal representations [38].Moreover, these measures differ in dimensionality.For instance, the unidimensional Philadelphia Geriatric Center Morale Scale (PGCMS; [39]) is one of the most common measures employed to assess attitudes towards own aging [37], albeit there is consensus about the construct multidimensionality [40].
Other relevant variables are age and the cognitive construct evaluated.Although there is evidence showing a decrease in cognitive efficiency that commences in the later years of middle age, there is some variability depending on the cognitive domain.For instance, speed of processing and inhibition start to show decreased efficiency during the early years of old age [41], and they secondarily affect some complex cognitive processes such as alternating attention and working memory update [42].However, domains such as language and simple attentional processes do not show this effect until reaching old age [43].The specific measures employed to measure such constructs may also affect the results since they might reflect subcomponents of the cognitive domain or not be the ideal candidate to measure it.In addition, there might be an interaction with an age effect on subjective aging since this variable accounts for a larger effect on midlife and young old samples than in elderly samples [19].
Finally, this review aims to examine potential moderator and mediator variables.Regarding physical mental health, several constructs have been proposed, such as selfefficacy [8], healthy behaviors [44], leisure activities [45] and loneliness [46].Since the stereotype embodiment proposes an effect through psychological, behavioral and physiological pathways, it is particularly relevant to address the variables through which subjective aging exerts an effect on cognition.One possible moderator variable regarding subjective aging and cognition is loneliness.There is substantial evidence about the detrimental effects of loneliness on cognition [47] and cognitive decline [48].Moreover, subjective aging seems to play a relevant role in the relationship between loneliness and other health outcomes.For instance, subjective age moderates the effect of loneliness and psychological symptoms [49], and loneliness moderates the relationship between self-perceptions of aging and depressive symptoms [46].Therefore, it is plausible that loneliness interacts with subjective aging through a psychological pathway, which, in turn, impacts cognition.Another candidate is depression.This condition has a notable effect on cognition and dementia risk [50], and it is a critical key point when diagnosing dementia [51].Moreover, subjective age is a significant predictor of depression [20], and that relationship seems to be unidirectional [52].Thus, it is possible that subjective age impacts depression, which, in turn, may affect cognition via behavioral and psychological pathways.Finally, there are reasons to consider physiological pathways.For instance, inflammation is a relevant variable for dementia [53], and it has shown a moderator effect between self-perceptions of aging and longevity [54].

Search Strategy
The main searches were conducted in PubMed, Web of Science and PsychInfo.Additional searches for grey literature were performed in Google Scholar, OpenGrey, WorldCat and NDLTD in order to reduce the risk of publication bias.Searches were last performed on 9th September 2023.The complete syntax employed in each database is available in Supplementary Data S1.
To determine all the constructs under the umbrella term of subjective aging, the first researcher detected the main terms for subjective aging based on Diehl & Wahl [30]: subjective age, age identity, self-perceptions of aging, attitudes towards own aging and aging self-awareness.Then, he searched for synonyms and related constructs, resulting in a total of 21 relevant terms.Most of the terms specified are included in recent systematic reviews and meta-analyses about subjective aging [19,20].The complete list of terms is available in Supplementary Data S1.This review employs the PRISMA system) and was registered in PROSPERO (CRD42023429916).

Inclusion and Exclusion Criteria
The inclusion criteria for studies in this systematic review are (a) ex post facto designs, (b) published in English or Spanish, (c) to include at least one measure of subjective aging, (d) to include at least one objective measure of cognition or cognitive decline and (e) to include a sample of 50 years old or elderly at any measurement point.These criteria try to ensure that all evidence regarding cognition and subjective aging is included, independently of whether it comes from a cross-sectional or a longitudinal design.Even if longitudinal designs are more suited to obtaining high-quality and semi-causal conclusions, evidence from cross-sectional designs is also valuable information.Moreover, the inclusion of samples of 50 years or elderly warrants that some normotypic changes in cognition associated with age are in play.Studies that (a) employed an ad hoc measure of subjective aging that is not described in detail, (b) measured objective cognition with an ad hoc measure that is not described in detail or (c) did not employ statistical analysis were excluded.

Study Selection
The study selection was performed using the Rayyan platform.First, OFB and MMM included all the studies found in the database searches and eliminated duplicates.Then, OFB and MMM independently selected potential candidates for inclusion by analyzing title and abstract.Disagreements were resolved through consensus.In a second phase, OFB and MMM independently selected the final articles included by performing a full-text review.Disagreements were resolved through consensus and assessment by a third author (JSG).

Data Extraction
Data extraction included (a) citation, (b) publication type, (c) purpose, (d) design, (e) sampling, (f) sample characteristics, (g) subjective aging construct and measures, (h) cognitive domain and measures, (i) moderators and measures and (j) results.The specific statistical results obtained (e.g., partial correlations, r2 adjusted, standardized regression coefficients, Cohen's d) are described in the Results section.

Quality Assessment
Quality assessment was performed using a modified version of the Newcastle-Ottawa Scale, available in Supplementary Material Data S2.It includes the following elements: (a) if the sample is representative of 50 years old or elderly population, (b) if the sample is extracted from such a representative population, (c) the presence of at least a measure of subjective aging, (d) the presence of at least a measure of cognition or neurodegenerative disease, (e) if its statistical analysis permits the examination of the relationship between subjective aging and cognition and (f) if it controls relevant variables through inclusion and exclusion criteria or by including them in the statistical analysis.Ratings vary between 1 and 6, 1-2 being an indicator of low quality, 3-4 medium quality, and 5-6 high quality.

Search Results
The results obtained and the study selection procedure are shown in Figure 1.A total of 4170 articles were identified in the original searches.After examining title and abstract, 90 were selected for a full-text review.After this phase, 31 papers were excluded: 2 because they did not include a proper measure of subjective aging, 3 due to not including an objective measure of cognition or cognitive decline, 16 since there was not enough information about measures employed and results obtained and the authors did not provide it after requesting, 6 because the publication was either a master's thesis, PhD thesis, poster or conference abstract and the published article was already included in this review, 2 since they did not report the statistical results for the link between subjective aging and cognition and 3 because the full text was not available in English or Spanish.A total of 59 publications were included after the final review round.The complete characteristics of the 59 publications included are shown in Table 1.
exclusion criteria or by including them in the statistical analysis.Ratings vary between 1 and 6, 1-2 being an indicator of low quality, 3-4 medium quality, and 5-6 high quality.

Search Results
The results obtained and the study selection procedure are shown in Figure 1.A total of 4170 articles were identified in the original searches.After examining title and abstract, 90 were selected for a full-text review.After this phase, 31 papers were excluded: 2 because they did not include a proper measure of subjective aging, 3 due to not including an objective measure of cognition or cognitive decline, 16 since there was not enough information about measures employed and results obtained and the authors did not provide it after requesting, 6 because the publication was either a master's thesis, PhD thesis, poster or conference abstract and the published article was already included in this review, 2 since they did not report the statistical results for the link between subjective aging and cognition and 3 because the full text was not available in English or Spanish.A total of 59 publications were included after the final review round.The complete characteristics of the 59 publications included are shown in Table 1.Cross-sectional ELES_PS • 1124 participants.
• Living in Spain.
Attitudes to aging (ad hoc) Self-perceptions of aging (ad hoc)
• No dementia point.No Yes Class 1 (many gains, few losses) showed better cognition compared with class 2 (moderate gains, few losses), 3 (many gains, moderate losses) and 4 (many gains, many losses) in all four cognitive measures.

Yes
Participants within the highest tertile of negative APQ had a higher probability of being classified in the cognitive decline trajectory (RRR = 1.82 after controlling for covariates.This link was partially mediated (40%), notably by loneliness (22%) and poor self-rated health (7%).The result was similar in the 65 years or elderly subsample if loneliness became the strongest mediator.(sd = 7).

Global cognition (MMSE) No Yes
There was a significant correlation between cognition and SPA (r = −0.175,p < 0.001).

Memory (word recall) No Yes
There was no significant association between own or partner's SPA and memory change at baseline or follow-up.
• Communitydwelling.There was an indirect effect of subjective age on memory performance through learning self-efficacy (b = 0.04).There was an effect of elderly subjective age on learning self-efficacy (b = 0.18) and an interaction of subjective age and education on learning self-efficacy (b = 0.33).After including covariables, more positive attitudes towards own aging baseline predicted less decline in fluid intelligence in men (βATOA = 0.71, p < 0.001) and explained 57% of the variance.However, it did not predict decline in women (βATOA = 0.7, p > 0.05).Positive attitudes did not predict less decline in crystallized intelligence.

Leisure activity and control beliefs Yes
More negative ATOA at baseline was related to higher risk of MCI or AD at time 3 (b = 0.283, p < 0.05; OR 1.43).

Cognitive decline (MoCA) No No
There were significant differences in the psychosocial loss subscale of the AAQ (t = 3.56, p < 0.01).Aging self-perceptions were not related to speed of processing (Es = 0.02, p > 0.05), memory (Es = 0.02, p > 0.05) or reasoning (Es = 0.00, p > 0.05) in the middle-aged sample.Aging self-perceptions were related to memory (Es = 0.07, p < 0.05) and reasoning (Es = 0.09, p < 0.05) in the elderly adults sample, but processing speed did not mediate this relationship.

Global cognition (MoCA) No Yes
There was a significant difference in the estimated marginal means of MMSE among 4 quartile groups after controlling for covariates (F = 13.122,p < 0.0001).Subjective age was associated with general cognition in the elderly after adjusting for covariates (b = 0.116, p < 0.0001).

Memory (picture recall and paired learning)
No Yes There was a significant correlation between views of aging and memory in the old group (r = 0.49, p < 0.01).
The direct path between views of aging and memory was significant (ES = −0.31,p < 0.001).The direct path between positive culture and memory was not significant (p > 0.05).
The direct paths between age and memory and age and positive age views were not significant (p > 0.05).

Participant Characteristics and Sampling
Regarding publication type, forty-nine of the studies included were articles, three were conference abstracts reported in scientific journals, three were master theses, three were PhD theses, and one was a poster.In terms of design, 35 publications were longitudinal, and 25 were cross-sectional.One of the publications followed a micro-longitudinal design with a duration of nine days and was considered cross-sectional in this review.Only seven studies reported specific analyses regarding differences between age subsamples.Quality assessment was made for all publications included, available in Supplementary Material Data S2.Fifty-three studies were rated as high quality and six as moderate quality.There were no publications rated as low quality.
The majority of studies (44) took data from representative databases for the elderly population, such as the Health and Retirement study (15), the Midlife in the United States study (six), the Interdisciplinary Longitudinal Study of Adult Development (five), the German Aging Survey (four), the Irish Longitudinal Study on Ageing (three), the English Longitudinal Study of Ageing (two), the PROTECT study (two), the Lothian Birth Cohort (one), the Activity and Function in the Elderly in Ulm (one), the Baltimore Longitudinal Study on Aging (one), the China Longitudinal Aging Social Survey (one), the Ageing in Spain Longitudinal Study (one), the National Health and Aging Trends Study (one), the Norwegian Survey of Health and Ageing (one), the Study on Global AGEing and Adult Health (one) and their own random and representative sample (one).Others employed data sources with a somewhat lesser degree of representation (one), such as the Dementia Literacy Survey (one), the Douglas Hospital Longitudinal Study of Normal and Pathological Aging (one), the Mindfulness and Anticipatory Coping Everyday study (one), the Subjective Cognitive Decline study (one) and the Population-based and Inspiring Potential Activity for Old-old Inhabitants study (one).Moreover, there was a proportion of publications that employed convenience samples (nine).One study combined a convenience sample with data from the Midlife in the United States study.

Effect of Subjective Aging on Cognition
The most frequent combination was the analysis of the link between subjective age and memory (16), followed by self-perceptions of aging and memory (9), subjective age and global cognition (6) and subjective age and executive functions (6).
From the 59 studies included in this review, 48 reported results for the relationship between subjective aging and objective cognition, 9 reported results for the link between subjective aging and cognitive decline and 2 reported analyses regarding subjective age and both cognitive function and decline.
A total of 42 studies that examined subjective aging and cognition reported at least one significant result supporting the hypothesized relationship, whereas 8 did not report any analysis that yielded a significant link.From these 43 studies, 25 reported significant results in all their main analyses, and 18 studies showed mixed results.Three studies reported differences regarding the subjective aging constructs and/or subdimensions employed, two in relation to cognitive domain differences and four due to the combination of both.Three studies reported mixed results after including covariables and mediators in their analysis, and one because of the combination of subjective aging dimensions and the inclusion of a mediator variable.There was also variability due to subsample analysis; one study reported it combined with covariate effects, one combined with subjective aging constructs and dimensions and two combined with cognitive domains.Additionally, one study reported a relationship in cross-sectional analysis that did not replicate longitudinally.
Analyzing the results obtained for specific combinations of subjective aging constructs and cognitive domains, the most replicated link found was between subjective aging and memory, with 15 out of 16 studies reporting a significant effect.A complete description of combinations of subjective aging and cognitive constructs and the results obtained are shown in Table 2.

Cognitive Domains and Measures
Most studies examined one (39) or two (13) cognitive constructs; it was rare to find reports that explored three or more cognitive variables at the same time (eight).Memory was the most common cognitive domain studied, being included in 29 reports, followed by global cognition (17) and cognitive decline (11).Word list recall (24) was the most frequent memory measure, the MMSE (8) for global cognition, and the TICS (6) for cognitive decline.Seven studies examined speed of processing, seven executive functions, six reasoning, four verbal fluency, four attention, three language, three fluid intelligence, two crystallized intelligence, two visuospatial abilities, two working memory and one a composite of processing speed, attention and executive functions.A complete description of cognitive domains can be found in Table 3, which accounts for considerable variability in the measures employed.

Subjective Aging Constructs and Measures
There was a high variability in subjective aging constructs and measures employed.Overall, 49 studies examined only one subjective aging construct, whereas 8 explored two constructs, 1 studied three variables, and 1 studied four variables.The most frequent construct was subjective age (26), followed by self-perceptions of aging (15), attitudes towards own aging (6), attitudes to aging (5) and awareness of age-related change (5).Aging satisfaction and age stereotypes were included in two studies, and aging expectations, age identification and look age were each examined in only one study.
Regarding the measures employed, all the studies that included subjective age (26) employed a composite score based on the age felt and the participant's age, although this measure was also employed in one study to measure views of aging.The second-most used measure was the unidimensional PGCMS (19), which was employed to evaluate attitudes towards own aging (seven), self-perceptions of aging (seven), age beliefs (three), satisfaction with aging (one), age stereotypes (one) and views on aging (one).Concerning multidimensional measures, the studies (six) that employed a variant of the AARC questionnaire studied awareness of age-related except for one, the studies (four) that employed any APQ variant measured self-perceptions of aging, and the studies (three) that used the AAQ explored attitudes to aging.One study studied self-perceptions of aging by using PEAS, and another by employing Age-Cog.One study used an ad hoc measure for age identification, and one used an ad hoc measure for look age.
Ten out of eleven studies that explored the link between subjective aging and cognitive decline reported at least a positive result in the expected direction.Seven showed positive results in all their main analyses, whereas three of them obtained mixed results because of covariates and mediators (one), differential effects between subjective aging constructs (one) and subsample variability (one).

Moderator Variables
Only nine studies included a moderation or mediation analysis.Seven studies that explored objective cognition included it, with a high variability in the constructs selected.Two studies included physical activity and depression.Activities of daily living, self-rated health, loneliness, smoking, alcohol consumption, social network, ageism, body mass index, learning self-efficacy, education, biomarkers, and social comparisons were each analyzed in one study.The only study regarding cognitive decline included leisure activity and control beliefs as moderators.

Discussion
Maintaining healthy cognition and reducing the incidence of cognitive impairment have become increasingly important due to population aging [110].This systematic review offers a synthesized view of the role of subjective aging on cognition and cognitive decline.There is a growing body of studies in this field, with 59 studies identified that were mostly published in the last ten years.The majority of results indicate a significant relationship between these constructs.More-positive self-perceptions of aging, younger subjective age and positive attitudes towards own aging are related to better cognition and reduced risk of developing cognitive decline or dementia.Moreover, there was consistency among reports, such as the link between memory and subjective age and self-perceptions of aging on global cognition.However, there were major sources of variation that need to be considered.
The first specific aim was to examine possible differences between subjective aging constructs.In this regard, most reports examined subjective age, a construct that reflects a discrepancy between felt age and chronological age.Even as this measure has been employed systematically in this field of study and is a significant predictor of cognition [20] and health and longevity [31], its unidimensional nature might reflect an oversimplified picture of subjective aging that does not account for variation regarding representation content [36].In this sense, several studies were identified that employed a multidimensional construct, such as attitudes towards own aging and self-perception of aging, which might be best suited to accounting for the complexity of this link.Additionally, some of these studies reported mixed results due to differential predictions and relationships, depending on the subscale employed.For example, in the case of awareness of age-related changes, Voelkner & Caskie [55] reported a link between cognitive losses and total losses but not of cognitive gains or total gains on memory, Zhu & Neupert [68] showed that total losses but not total gains are related to reasoning and Sabatini et al. [36] reported that cognitive losses and total losses but not gains were related to working memory.In the case of self-perceptions of aging, Robertson et al. [93] indicated that positive control and negative control are related to verbal fluency, whereas the timeline subscale was associated with prospective memory.This effect may also interact with chronological age, as Jung [94] pointed out that the social loss dimension of self-perceptions of aging predicts cognition only in a subsample of elderly adults, and Sabatini et al. [36] indicated that the cognitive gains dimension of awareness of age-related change predicted working memory and reasoning only in middle-aged and young old participants.All this evidence leads to the notion that some dimensions are best suited to accounting for certain effects on cognition.
Another objective was to identify the measures employed to address multidimensional constructs of subjective aging.Whereas some studies employed complex questionnaire measures, there was a high number of reports that employed the PGCMS for this task.It has been pointed out that this scale might not be best suited because of its unidimensional nature and unspecific item content [37].Moreover, there was a significant variability regarding the construct measured, having been employed to explore attitudes towards own aging, self-perceptions for aging, age beliefs, satisfaction with aging, age stereotypes and views of aging.In order to increase the clarity and specificity of the umbrella concepts under the term of subjective aging and to better operationalize the measures employed, it is relevant to reduce this variability and commence using validated scales that respect the dimensionality and content of the constructs.Therefore, we recommend the use of the PGCMS only when assessing morale from a unidimensional point of view and the use of well-established questionnaires such as the APQ and the AARC for subjective aging.
The third aim was to examine variation regarding the cognitive constructs evaluated.Some of them seem to not replicate to the full extent along the reports, such as subjective age on global cognition and self-perceptions of aging on memory.Examination of studies that included more than one cognitive domain also revealed mixed outcomes; self-perception of aging seemed to predict executive function and memory, but not attention [92], and subjective age predicted memory but not executive function [82].There can be several reasons for this variation.First, there is substantial variability in the neuropsychological measures used to assess cognition.For instance, executive functions were measured by the single use or combination of nine tasks that might reflect very distinct subcomponents, from category fluency to working memory and reasoning.This reasoning might also apply to language since it was operationalized with vocabulary, naming and fluency tasks that differ in the processes involved.A more salient case is global cognition, which included a variety of 10 tasks, the most common of which was the MMSE.As it has been pointed out, this measure might not be best suited to addressing the variability of normal cognition since it only captures modest age-related changes when compared to other neuropsychological tasks [111].Therefore, we suggest that a more process-based approach may be beneficial for this field since it permits accounting for specific subcomponents of cognition and uses, to a lesser extent, a general construct such as global cognition.A second source is the covariates controlled in statistical analysis.For example, Morris et al. [71] found that subjective age predicted memory, executive function, language and speed of processing but that the effect after including depression only remained significant for language and speed of processing.In this regard, it is especially important to include and statistically control potential confounding variables.Finally, there might be an interaction with age.Some of the reports found differences when analyzing age subsamples on global cognition [80,94], working memory [36], memory, reasoning [36,106] and speed of processing [106].This suggests that subjective aging might affect specific cognitive domains more profusely depending on the sample age, especially if it matches the neurotypical cognitive trajectories.
This review has several limitations.The inclusion of both cross-sectional and longitudinal designs encompasses a great amount of available evidence, but the latter are more appropriate for drawing conclusions about cause and directionality.Even if the inclusion of high-quality cross-sectional designs is valuable for examining the relationship between subjective aging and cognition, their results are to be carefully interpreted.Moreover, it does not offer a meta-analytic approach to better understand the effect sizes.
We suggest that future studies focus on longitudinal designs that include measures with strong psychometrical properties to measure subjective aging.In this regard, efforts could be directed to extending the evidence about questionnaires that have shown promise, such as the AARC and the APQ, and to avoiding using others that cast doubts, such as the PGMCS.Additionally, it might be of special interest to examine the differential effects of such constructs on cognition and whether some of them show stronger associations.Regarding cognition, we suggest a more detailed and process-based neuropsychological approach that employs sound tasks for specific cognitive domains or systems.Therefore, employing ad hoc measures or screening tools of general cognition is to be avoided.This process-based approach also needs to account for the specific domain that is more salient in each neuropsychological task.For instance, the picture completion task might be better suited to assessing selective attention rather than reasoning.Additionally, it is of special interest to increase the available evidence for some domains that are key for adult cognition and are underrepresented in the studies included, such as speed of processing, visuospatial abilities and working memory.This proposal also applies to cognitive decline and dementia, being relevant to ensuring that the selection of participants of this population or the splitting of a sample between people with and without cognitive decline follows the international criteria.Finally, we recommend including potential moderators for this relationship since only nine studies directly addressed it, and there was high variability in the moderator variable included.It is relevant to replicate the available evidence of studied moderators such as ageism [55], depression [71], loneliness [62], leisure activities [87] and activities of daily living and social network [78], and also to include other relevant variables such as optimism [17].

Conclusions
Subjective age is a relevant variable when predicting and explaining cognition and cognitive decline in old age.More-positive self-perceptions and attitudes towards own aging and a younger subjective age are related to better cognition and lower risk of cognitive decline.However, several variation issues need to be addressed in future research.It is important to create evidence for each of the umbrella terms included under subjective aging since they might affect cognition differentially.Moreover, these constructs need to be measured with psychometric questionnaires with solid evidence about their content and structure validity, and the same scale should not be used for different constructs.A more precise and process-oriented neuropsychological approach may be of great benefit since the measure selection and the sample age are potential sources of variation.Additionally, it is important to increase the evidence regarding key cognitive domains for the elderly population, such as speed of processing and working memory.Finally, it is relevant to identify and test potential moderators in this relationship to better understand the pathways implicated.

Table 1 .
Full description of studies included.
• Mean age of 70

Table 3 .
Cognitive domains and measures.