According to the latest United Nations Educational Scientific and Cultural Organization (UNESCO) report on global literacy, there are 114 million illiterate adolescents and youths (15 to 24 year olds) around the world, two-thirds of whom are female [1
]. Despite widespread acknowledgement of this problem, between 2000 and 2015, global literacy rates were estimated to have improved by just 4%. Studies have shown that poverty has a considerable impact on psychological functioning and that, for children, this impact increases in severity the longer they live in poverty [2
Progress in the Philippines to address national literacy rates has been slow [5
]. The literacy rate of individuals aged 15 to 24 years improved from 93.4% in 2003 to 98.1% in 2013 [6
]. This improvement is likely to be due to increased school opportunities, education expansion, better access to barangay (small administrative divisions in the Philippines) health stations, improvement in general health status, decreased poverty rate, and reduced burden of diseases [8
]. However, this may also partly be due to measurement errors, and differences in survey designs and tools used by the national statistics office in different years [10
]. Despite the overall improvement, there are areas in the Philippines where functional literacy remains low [11
There are numerous measurement tools available to assess different domains of cognitive function at different stages of childhood [12
]. One indicator at the population level is functional literacy. Although the meaning of functional literacy varies between countries, regions, and cultures, it is generally understood to be the ability to use a set of cognitive skills to engage in reading, writing, and numeracy for effective functioning and development of individuals and their communities. Conversely, functional illiteracy is therefore defined as the inability to use cognitive skills to engage in reading, writing, and numeracy [14
Previous studies have demonstrated a clear link between literacy and cognitive abilities [15
]. Cognitive development of children is influenced by the interplay of several factors. Evidence from longitudinal studies in children indicates that neurocognitive development is impacted by nutritional status, poverty, stress, low maternal education attainment, less nurturing child-rearing environments, younger maternal age, living in a rural environment, and poor psychosocial stimulation at home and at school [17
]. Cognitive processes that result in functional illiteracy can be influenced not only by individual and household factors but also by environmental determinants. Cognitive impairments are multifactorial in nature and both non-communicable (e.g., malnutrition) and communicable diseases (e.g., malaria and neglected tropical diseases) are known contributors [20
]. The exposure to many of these conditions is determined by anthropogenic environmental factors (i.e., human impacted habitat conditions) such as sanitation, urbanisation, and overcrowding [21
], and natural environmental factors such as elevation, temperature, and water availability [22
Functional literacy indicators are likely to vary between locations given the geographical variability of its major determinants. This property poses a challenge to decisions around efficient allocation of population services and resources to mitigate the impact of functional literacy in populations most in need. The application of spatial epidemiological approaches has been useful for identifying areas where the risk of diseases is at its highest and for highlighting areas where interventions are most needed. Spatial epidemiological approaches have been utilised to address such challenges globally [22
], and in the Philippines, mainly looking at infectious diseases such as soil-transmitted helminths [25
]. However, at present no study has attempted to investigate the spatial epidemiology of functional illiteracy and quantify the role of different determinants in functional illiteracy. Such a study would provide an evidence base for understanding local burden of functional illiteracy and assist in geographical targeting of interventions.
In this study, we aimed to quantify geographical disparities in functional literacy in the Philippines and the role of socioeconomic status (SES), water supply, sanitation and hygiene (WASH), household education stimuli, and environmental variables in the observed geographical disparities.
Functional literacy is a key indicator of cognitive function, especially information processing and comprehension, and has been used to measure cognitive function in school-aged children in previous studies [15
]. Evaluation of geographical determinants of functional literacy is critical for designing and implementing spatially targeted interventions which could contribute to efficiently attaining the Sustainable Development Goals (SDGs) for functional literacy in school-aged children in the Philippines [38
This study represents the first nation-wide population based spatial epidemiological study investigating geographical disparities in functional literacy indicators and its association with individual-level variables (sociodemographic factors), household-level variables (SES, WASH, household education stimuli factors), and environmental variables (DPWB, LST, NDVI, rainfall). Our study indicates that the prevalence of functional illiteracy is heterogeneous in the Philippines while showing important regional differences in key determinants of functional illiteracy.
4.1. The Role of Household WASH and Educational Stimuli in the Spatial Variation of Functional Illiteracy in Luzon
Household WASH variables including type of toilet facility and the main sources of drinking water were found to be important determinants of the prevalence of functional illiteracy in the region of Luzon. Households in Luzon without access to toilets had higher prevalence of functional illiteracy compared to households with flush toilets.
Previous studies have demonstrated that practice of open defecation increases the risk of childhood stunting, and transmission of infectious diseases [39
]. Available evidence suggests that households with access to unprotected drinking water sources are at increased risk of infectious diseases such as cholera, and other bacterial and infectious diarrhoeal diseases which are also known to affect children’s development [41
]. Our results show that the use of unprotected drinking water sources (lake, pond, rainwater or rivers) is also associated with reduced functional illiteracy.
Our findings may be confounded by the fact that unprotected drinking water sources are more likely to be present in agricultural communities where access to food and nutritional security are maintained through local food production. Natural rivers and lakes are often used as sources of agricultural irrigation and water for livestock [43
]. Indeed, previous studies indicate that food security has a positive impact on nutritional status and the long-term cognitive development of children [44
]. Further investigation is needed to examine the factors mediating the relation between access to water sources and the prevalence of functional illiteracy identified in this study.
Our results also demonstrate that in Luzon, the quality of the home environment is an important predictor of functional literacy of school-age children. Indeed, our results showed that of the three regions, Luzon had the lowest proportion of households classified as poor and that the average total scores of households’ education stimuli were higher in Luzon compared to the Visayas and Mindanao. This may mean that household education stimuli available to children in Luzon reflects not only household SES but that there is variability of household education stimuli scores within SES classes. Previous studies also found house environment mediated the association between family SES and executive functions of inhibitory control and working memory in school-aged children [45
Taken together, our findings suggest that functional literacy in Luzon may benefit from health promotion interventions that improve personal hygiene practices, which could play a key role in mediating the effects of infectious diseases and malnutrition, and assist families with providing a stimulating environment for children of school age.
4.2. The Role of Environment and Socioeconomic Status (SES) in the Spatial Variation of Functional Illiteracy in the Visayas and Mindanao
Environmental factors and poverty play important roles in the spatial variation of functional illiteracy in the Visayas and Mindanao. While our results indicate that environmental determinants play different roles in functional illiteracy depending on the region, the effects of environmental variables on the spatial variation of functional illiteracy were greatest in Mindanao.
The Philippines frequently experiences droughts, typhoons, and flooding which lead to the destruction of crops, land degradation, and siltation of irrigation systems caused by severe erosion [46
]. Such extreme weather events are of concern particularly among households in Mindanao where agriculture and fisheries are the major economic activities. Evidence suggests that since early 2000 extreme weather events including long periods of high temperatures and intense rainfall have intensified in Mindanao [47
]. Farming communities are particularly vulnerable to climate-associated food insecurity which has a negative impact on the nutritional status and child development in affected populations, as well as increasing the susceptibility to disease [44
Our findings showed that low SES was positively associated with functional illiteracy in Mindanao. Evidence suggests a possible link (behavioural and neurobiological) between low SES and functioning of different domains of neurocognitive systems including children’s performance on language and literacy skills [19
]. This relation is mediated by different mechanism such as parenting behaviour, linguistic stimulation and children’s experience of stress [19
The observed association could also be explained by the effect of malnutrition in the poorest areas of Mindanao. Our map of the spatial distribution of functional illiteracy in Mindanao (Figure 1
) shows a higher rate of functional illiteracy in western Mindanao, known for its high prevalence of underweight, stunting, and wasting in children [49
]. Furthermore, Mindanao has suffered social and political conflict for many years which have led to greater food insecurity, limited land use options, and a higher rate of poverty in this region [47
]. Further investigation is needed to understand the factors that mediate the association between SES and functional illiteracy in Mindanao and the Visayas.
Taken together, our findings suggest that poverty is likely to be a key determinant of malnutrition in this region, explaining the geographical heterogeneity of functional illiteracy indicators. Areas with high rates of poverty may benefit from integrated interventions that aim to reduce malnutrition in school-aged children.
The results of this study need to be interpreted in light of several limitations. Our estimates of functional illiteracy indicators rely on performance-based functional literacy data. Although these tools are designed to measure different domains of cognitive functioning in school-age children, performance-based measurement tools may be differentially related to the outcome—for example participants who completed self-reported questionnaires may have had a chance to ask interviewers questions and get support when required [51
Further, our data are from the 2008 FLEMMS survey and may not reflect the current situation. However, these data constitute the most comprehensive and up to date information on functional illiteracy in the Philippines and yield novel insights into the ways environmental input can affect child development. Data on literacy rates are scarce due to the costs involved in their collection and processing [8
] and in the Philippines, census data are collected every ten years [7
]. The latest data on literacy were collected in 2010, however information on functional literacy rate has not been updated in the report produced by the Philippines government since 2008 [6
]. It is notable, however, that the rate of functional literacy has not seen much improvement in the last three FLEMMS surveys (83.8% in 1994, 84.1% in 2003, and 86.4% in 2008 (data from 1989 was not available for comparison)) [7
] and therefore we do not expect much difference in the current situation.
In addition, we used an ecological approach, using secondary data on environmental predictors of functional illiteracy such as climate and SES [44
]. Some of these proxies are imprecise measurements of exposure, resulting in regression dilution bias leading to underestimation of the observed effects [53
Further, we created our own home inventory-proxy measurement (education stimuli measure), so our results cannot be readily compared with the standard home-inventory index questionnaire [54
]. Our education stimuli measure included selected variables in the FLEMMS survey, which attempt to document whether access to learning materials and activities would improve cognitive function of school-aged children [57
]. Nevertheless, our results indicated that there is evidence for a degree of specificity of the effect of household education stimuli. Our education stimuli score appeared to be a reasonably reliable scale with moderate correlations with SES element, which was based on ownership of household amenities (a pairwise correlation coefficient of 0.36).
Finally, our paper highlights the need of a complementary modelling approach that could be used to investigate potential reverse causation. While standard multivariate regression models may be useful for exploring the effects of predictor variables on the outcome variable, it may not be sufficient to explain the interrelation (associations and dependency) between multiple interdependent variables [60
]. Previous studies showed that maternal literacy can improve the health navigation skills of mothers, which leads to child mortality reduction [34
]. Multiple determinants of literacy have variable impact depending on where and how they are embedded in a child’s life. Future studies need to account for the complex interdependencies between these determinants.