This study revealed noteworthy SES-related disparities in neurocognitive development among preschool-aged children. It is well known that children born with low SES are at an increased risk of neurocognitive delay by 6 years compared to those born with high SES.
Cognitive and language skills emerged as salient areas of distinction, although disparities were evident across all developmental domains. This aligns with the existing research that underscores the substantial influence of SES on language and cognitive development, among other facets of child development. Our findings corroborate the well-established notion that SES has a robust impact on various domains of child development. This influence persisted even after considering several influential variables, including sex, BW, HC, birth residence, parental income, year of birth, and breastfeeding status. By scrutinizing the impact of these variables on the development of our cohort, we observed parallels with previous research, such as sex, while uncovering divergences exemplified by the role of breastfeeding.
We also investigated the timeline at which SES-related developmental delays became apparent. Intriguingly, disparities were discernible as early as 9–12 months of age, coinciding with the initiation of developmental screening, and became more apparent by 18–24 months of age. This temporal analysis provides valuable insights into the trajectory of SES-related developmental differences, thereby enhancing our understanding of the critical periods during which interventions may be most impactful.
Elucidating the association between socioeconomic factors and children's development has been a persistent subject of inquiry. Substantive evidence underscores the impact of SES on overall brain development and children's behavior,23 particularly in domains such as executive function and memory ability.24–27 These disparities, well-documented in the literature, are attributed to increased behavioral problems and diminished self-regulation in early childhood contingent upon SES.28 The relationship between SES and cognitive development transcends geographical boundaries and economic disparities, as corroborated by extensive global statistical confirmation.29–31
Notably, lower SES has been linked to a higher prevalence of intellectual deficits among children, as evidenced the study by Chowdhury and Gosh30 examining 500 children, where lower SES was associated with intellectual deficits measured through nutritional status. In post-transitional countries, SES has been identified as a pivotal determinant of developmental outcomes, even among children of average weight.31 Cognitive developmental delays appear to manifest as early as infancy, with discernible differences in vocabulary and picture similarity.32 These early disparities, particularly in language and cognitive domains, are precursors to broader discrepancies, including lower intelligence quotient (IQ) and divergent academic performance.4,33 The pervasive influence of SES extends to language development, impacting IQ at younger ages and influencing educational achievement in the early school years.
Empirical evidence from studies conducted in the United Kingdom underscores the elevated risk of language delays among children from lower economic backgrounds. Our findings of disparities in cognitive and language development during the preschool years align with the broader body of evidence.34 Critical facets of cognitive and language development influenced by SES include parental education, home environment, caregiver–child relationships, language exposure, and interactive communication dynamics. It is noteworthy that early language development plays a predictive role in later academic performance, underscoring the importance of parent–child communication.
Neuroimaging studies revealed distinct brain activity patterns in children with high- and low-SES, elucidating the neural underpinnings of cognitive and language skill disparities. Children with high SES exhibit increased activity in brain regions associated with math and language performance, while children with low SES exhibit heightened activity in areas linked to spatial processing.35
Sex-specific neurocognitive developmental outcomes have been extensively investigated, drawing insights from epidemiological, biological, cognitive, neurobiological, genetic, endocrinological, and immunological studies. The multifaceted influences of sex and sex-related attributes on neurocognitive development are shaped by intricate interactions involving biological, psychological, and sociocultural factors.13 Reduced fetal growth, as indicated by BW, is not considered a direct etiological agent in neurodevelopmental delay but rather serves as an indicator of broader phenomena, such as fetal malformations, intrauterine inflammation, and conditions such as pre-eclampsia and gestational hypertension, which are more plausible contributors to neurodevelopmental challenges.14 Furthermore, HC at birth serves as a surrogate measure for prenatal cerebral growth, with smaller cranial dimensions indicating an augmented risk of subsequent neurodevelopmental issues.15
Long-chain polyunsaturated fatty acids in breast milk play a pivotal role in facilitating optimal brain development, as evidenced by consistent research highlighting the advantageous impact of breastfeeding on neurological development and superior academic performance. Empirical evidence supports the increased neural volume and activity in critical brain regions associated with neurological function in breastfed children. Additionally, breastfeeding fosters an enhanced mother–child bond, exerting a beneficial influence on neurodevelopment.16 Other factors influencing neurodevelopment, such as atopic dermatitis, torticollis, and soy milk feeding, merit consideration in future studies and subgroup analyses.36–38
The outcomes presented in this study affirm the substantial influence of SES on developmental trajectories, underpinned by diverse contributing factors, including protective mechanisms and maternal dietary considerations. Notably, findings regarding breastfeeding diverge from conventional wisdom. While breastfeeding is acknowledged for its myriad advantages, a nuanced perspective must be entertained, exercising cautious optimism regarding its role in preventing developmental delays. However, this optimism is tempered by the conjecture that breastfeeding efficacy may be influenced by intricate mechanisms related to micronutrient availability and maternal nutritional status.
Developmental screening serves the critical purpose of identifying developmental delays at the earliest possible juncture and facilitating timely interventions. The present study found an elevated prevalence of developmental delays in lower-SES groups at an earlier age, accentuating the potential societal advantages of directing attention toward developmental delays in these cohorts. Countries such as South Korea and the United States have instituted comprehensive child screening systems that integrate developmental screening protocols. In the United States, developmental screening is recommended at 9, 18, and 30 months of age. While the overarching objective of this screening initiative may not be universally acknowledged, its primary objectives include augmenting the likelihood of detecting developmental delays through standardized instruments, expediting referrals for specialized assessments.39
As this study revealed SES-related disparities in the probability of developmental delays from early childhood, it underscores the potential benefits of advocating for more thorough follow-up assessments or in-depth evaluations for children in the low-SES category. This proactive strategy, extending beyond mere developmental surveillance, is of paramount importance for enabling early interventions and mitigating the protracted societal ramifications associated with developmental delays. Global implementation of such screening programs underscores their significance in addressing developmental delays and fostering timely interventions to improve long-term outcomes.
This study had the advantage of encompassing an extensive cohort, representing almost all children born in a single country, thereby ensuring a substantial and minimally biased sample. The prolonged follow-up duration, spanning approximately 6–7 years into the preschool period, facilitated a comprehensive longitudinal assessment of developmental changes. Robust statistical corrections for various known variables were implemented to bolster the reliability of the findings.
However, some limitations of this study warrant further consideration. Although the study was based on extensive datasets with multiple calibration variables to mitigate selection bias, the inherent lack of numerical precision of the developmental test posed a methodological challenge. The absence of developmental data from birth to the final assessment at 66–71 months imposed constraints on the comprehensive tracking of developmental sequences. The utilization of SES values at birth may not have precisely captured changes over the 6-year study period. Additionally, reliance on parental recall for breastfeeding records introduced potential recall bias, and the transition from the K-ASQ to the K-DST as a developmental screening test may have influenced accuracy. In the absence of K-DST data, the study resorted to the K-ASQ, resulting in numerical inconsistencies despite efforts to align the results closely. Notably, the study overlooked key educational measures, such as day care or preschool attendance, which play a pivotal role in the comprehensive assessment of child development.