Background
Early childhood has been identified as an important window for cognitive development [
1‐
5]. It is well known that cognitive development during the first 3 years of life is predictive of cognitive development in later childhood [
6‐
9]. Theoretical and empirical research has established that basic cognitive skills developed in the earliest years of life form the foundation for the development of more complex skills later in childhood [
4,
10,
11]. For this reason, infants who have delayed development during the first 3 years of childhood face impediments to developing more complex cognitive skills and are likely to continue to have low levels of cognitive skill into their preschool years [
6,
7,
12‐
15]. Several studies have found that cognitive skills at age 3 were predictive of cognitive skills at preschool age (4 to 5) with coefficients of 0.36 to 0.64 [
6,
8,
16].
Although the literature has consistently found that levels of cognition are relatively constant after a child is 3 years old [
17,
18], a number of studies, especially in developed countries, have found that fluctuations in cognitive development before age 3 are relatively common [
6,
19‐
24]. For example, a study by Feinberg et al. [
21] found that 28% of cognitively delayed children in the United States were persistently delayed from 9 to 24 months old, while the remaining children who were delayed at 24 months (72%) were newly delayed (i.e., their levels of measured cognition had deteriorated between 9 months and 24 months). In another study from the United States, nearly 70% of sampled children with measured cognitive delay at 9 months had recovered to normal cognition by 24 months [
23], while the remaining 30% of children remained delayed at 24 months [
23]. Although the changes in cognitive development over time may be due to measurement error, the measurements used in these studies have high reliability, and most of the changes are likely to represent true fluctuations in cognitive development.
A small number of studies have provided evidence that trajectories of cognitive development before 3 years of age affect later development, such as behavioral outcomes, at preschool age. To the best of our knowledge, such studies have been conducted only in developed countries [
19,
23]. One study conducted in the United States, for example, demonstrated that different cognitive trajectories before 3 years were linked to differences in preschool-aged developmental outcomes [
19]. Compared to children who never experienced cognitive delay (at age 5), persistently delayed, improving, and deteriorating children were shown to have higher frequencies of behavioral problems by 0.6 standard deviations (SD), 0.2 SD, and 0.4 SD, respectively. Unfortunately, however, the study did not examine whether the trajectories of cognitive development in early childhood were correlated with cognitive outcomes at preschool age.
Understanding the trajectories of cognitive delay and their relation to later cognitive skills should be especially relevant for developing countries, where research has shown that about 250 million children under the age of 5 (about 43%) are at risk of developmental delay [
25]. Of this global total, it is estimated that 45 million are in China, which would make China rank second globally in terms of total number of young children with cognitive delay [
25‐
27]. In China, recent studies suggest that cognitive delay is most prevalent among children in rural areas. Whereas research in urban areas has consistently shown rates of cognitive delay among infants and toddlers of under 15%, the average rate of delay for a healthy population [
28‐
30], studies of children aged 0 to 3 years in rural China have found rates of cognitive delay between 39 and 49% [
26,
31,
32]. Although fewer recent studies have examined the cognitive development of preschool-age children in rural China, the existing studies have found that the rates of cognitive delay of preschool-age children are similarly high: a 2008 study of 505 low-income, rural children aged 4 to 5 found a rate of cognitive delay around 57% [
33], and a more recent study conducted in Guizhou province in 2015 found that over half of rural children aged 64–71 months were cognitively delayed [
34].
Although previous studies in rural China have not focused on the trajectories of cognitive skills, there is evidence that suggests that cognitive development children under age 3 in rural China may fluctuate over time. A longitudinal study of young rural children in China found the rate of cognitive delay of sample children increased from 14% at 6 months to 49% at 29 months [
35]. Another study, which examined the impacts of a parental training intervention to increase psychosocial stimulation, found that it was possible to improve the cognitive development of the sample children, especially those children who had low levels of cognition at baseline [
36]. Importantly, however, no study in China to date has documented the trajectories cognitive development among young children under age 3. Additionally, little is known about how different trajectories of cognitive development before age 3 may predict cognitive development as children grow older (e.g., to preschool age).
If the trajectories of cognitive development during 0 to 3 years of age are associated with development in later childhood (both preschool age and beyond), there would be great value for researchers and policymakers to determine the factors linked with the different trajectories. To our knowledge, however, only one study has examined factors associated with the different trajectories of cognitive development among young children [
19]. This study, conducted by Cheng et al. (2014) among children in the United States, found that trajectories of cognitive development from infancy to preschool age were related to the demographic characteristics of children and families: children with low birth weight and those from families with low income were more likely to experience persistent cognitive delay, whereas female children and those with siblings were more likely to see their cognitive development improve. Additionally, although no studies in developing countries have examined cognitive trajectories before age 3, many studies in developing counties have identified risk factors that are correlated with early childhood cognitive delay, including low parental education levels, low family income, and greater number of siblings in a family [
26,
35,
37,
38]. Given the consistency of these findings across settings as diverse as Columbia, China, and South Africa, it is possible these factors also may be related to trajectories of child cognitive development.
The aim of this paper is to describe the trajectories of child cognitive development between the ages of 0–3 years among in rural Western China and examine how different trajectories predict cognitive development at preschool age. To achieve this goal, we have four specific objectives. First, we describe child cognitive development at three points in time: infancy (6–12 months), toddlerhood (22–30 months) and preschool age (49–65 months). Second, we describe the trajectories of child cognitive development from infancy to toddlerhood and report the shares of sample children who are never delayed, persistently delayed, show improving cognitive development and show deteriorating cognitive development before age 3. Third, we examine how the different trajectories of cognitive development before age 3 predict cognitive skills at preschool age. Finally, we identify individual and household factors that are associated with each developmental trajectory before age 3, including the child’s age, gender, whether the child was born prematurely, whether the child had siblings, maternal age, maternal education level, and the family asset index of each household.
The remainder of this paper is structured as follows. Section 2 presents our methods, including sample selection, data collection, and statistical methods. Section 3 describes the results. Section 4 discusses the findings, and section 5 concludes.
Discussion
We studied the trajectories of child cognitive development before 3 years of age in rural Western China and examined how these paths affect predict cognitive skills at preschool age. We described the cognitive development outcomes of children when they were in infancy (6–12 months), toddlerhood (22–30 months), and preschool age (49–65 months) and identified children who were never delayed, persistently delayed, had improving cognition and had deteriorating cognition before age 3. The empirical analysis also examined the associations between trajectories of cognitive development before age 3 and cognitive development skills at preschool age and identified risk factors (child and household characteristics) associated with each trajectory of cognitive development before age 3.
The results demonstrate that the prevalence of cognitive delay among rural infants (20%), toddlers (55%), and preschoolers (45%) is significantly higher than what one would expect for children in a healthy population (15%) [
47,
55]. These findings are consistent with a number of recent empirical studies in rural China [
26,
31‐
34,
56]. According to these studies, 39 to 49% of infants and toddlers between 6 and 36 months are cognitively delayed, and 37 to 57% of children at preschool age are cognitively delayed. Hence, our results, using three observations for the same cohort, concur with the cross-sectional studies in the literature, indicating that the cognitive delay of children during the first 5 years of life is a common problem across rural China.
The data also revealed that a large share of children had deteriorating cognitive development before age 3. Whereas only 13% of children had persistent cognitive delay, 41% of the sample saw their cognitive skills deteriorate, meaning that they developed cognitive delay as they aged from infancy (6–12 months) to toddlerhood (22–30 months). In contrast, only 7% of the sample children saw their cognitive skills improve (recovered from cognitive delay between infancy and toddlerhood). These findings suggest that sample children in rural China who were cognitively delayed in infancy (20% of the original sample) were less likely to recover from cognitive delay by the time they reached toddlerhood. Moreover, over half of the children who were not cognitively delayed in infancy became delayed by the time they reached toddlerhood.
Perhaps most importantly, the analysis demonstrates that different trajectories of child cognitive development before age 3 predict different levels of cognitive skills at preschool age. Children who were never cognitively delayed and children with improving cognitive trajectories had significantly higher levels of cognitive skills when they reached preschool age, whereas children who were persistently delayed and those with deteriorating cognitive trajectories during the first 3 years had relatively lower levels of cognitive skills at preschool age. Although there has not been a lot of work in this specific area, the findings are in line with at least two previous international studies [
19,
23], which found that children who exhibit cognitive delay in early life (at 9–24 months old) have a higher likelihood of being cognitively delayed later in life (at 4–5 years old). The finding that “never” delayed and “improving” children in the sample show similar levels of cognition at preschool age indicates that identifying and addressing cognitive delays before age three may reduce the overall prevalence of cognitive delays and promote healthy long-term development among children in rural China.
In addition, we were interested in which measure was most predictive of cognitive development at preschool age: a child’s cognitive trajectory before age 3, cognitive development at infancy (6–12 months), or cognitive development at toddlerhood (22–30 months). The results indicate that the cognitive trajectory before age 3 has similar predictive power to a child’s level of cognitive development at 3 years. Although no study has considered this issue specifically, other studies have shown that a child’s level of cognition at 3 years predicts cognitive skills when a child is 5 years old. Specifically, research [
6,
16,
57] has shown that the predictive power, measured as
R-squared (goodness of fit) of the equation, using 3-year-old cognitive development to predict 5-year-old development, ranged from 0.36 to 0.77. More importantly, the finding that the prediction of the cognitive trajectory before age 3 to cognitive development at preschool age is the same as that of cognitive development at age 3 suggests that it may not be worth spending valuable resources to monitor the trajectory of child cognitive development unless the monitoring is helpful in inducing investment in children that would arrest deterioration and overcome the persistence of cognitive delay to enable young children to improve their trajectory. In the case of rural China, however, where our study finds 41% of children have deteriorating cognitive development before age 3, monitoring developmental trajectories in early childhood may help to identify vulnerable children and provide timely intervention.
Finally, we identified a relatively small number of individual characteristics associated with the socioeconomic status of the caregiver that predict improving or deteriorating cognitive trajectories before age 3. Children who had older mothers, more educated mothers, and lived in households with high family asset indices were less likely to experience deteriorating trajectories of cognitive development and were more likely to experience improving trajectories. Such a finding is consistent with previous international research that has investigated factors associated with child cognitive development at a single point in time [
58‐
62]. The research found that older mothers, more-educated mothers, and higher socioeconomic status of the household were positively associated with better child cognitive development. For example, a study conducted in Ecuador in the early 2000s, using a sample of 3000 children aged 36 to 72 months from poor families found that household wealth and maternal education were associated with higher cognitive scores [
62].
This study makes three contributions to the literature. First, the strengths of this study include its population-based sampling technique, large sample size, and rigorous child development testing, all of which increase confidence in the validity of our findings. Second, this is the first study to investigate the trajectories of child cognitive development and the association between these trajectories and preschool-age development in rural China; it is also one of only a few studies to do so internationally. Finally, this study examined factors associated with the different trajectories of child cognitive development in rural China, and these findings may provide specific indicators to target the children who are more vulnerable to delayed cognitive development. This information also may help researchers and policymakers to improve the interventions aimed at reducing the prevalence of child cognitive delay in the early years of life.
We also acknowledge two limitations of this study. First, although we document changes in cognitive development from infancy to preschool age, the data were collected in three survey waves, separated by intervals of nearly 2 years. As a result, this analysis may underestimate the true share of children who were affected by cognitive delay through early childhood. Second, although the samples in this study were randomly selected from the Qinba Mountain area of China, we do not consider our results to be statistically representative of the entire country or other rural regions. Future studies should examine the changes in cognitive development of early childhood over shorter intervals to better understand the trajectories of cognitive development in both the short and long terms. Moreover, future studies should continue to expand on the current study by using surveys of a wider scope and sampling populations from other rural areas in China that this study did not explore.
Conclusion
We studied the trajectories of child cognitive development before 3 years of age in rural Western China and examined how these paths predict cognitive skills at preschool age. Drawing on longitudinal data from 1245 children and their families in 11 rural counties in Western China, the results found that 20% of children were cognitively delayed in infancy (6–12 months), 55% were delayed in toddlerhood (22–30 months), and 45% were delayed at preschool age (49–65 months). About 41% of children had a deteriorating cognitive trajectory from infancy to toddlerhood, whereas only 7% had an improving trajectory. Compared to children who had never experienced cognitive delay, children with persistent cognitive delay and those with deteriorating development before age 3 had significantly lower cognitive scores at preschool age. Children with improving development before 3 showed no similar levels of cognition at preschool age as children who had never experienced cognitive delay. Children with older mothers, children whose mothers had higher education levels, and children from families with higher asset index scores were less likely to have deteriorating cognitive development and more likely to having improving cognitive development before age 3.
Findings of this paper have clear implications for both policymakers and researchers. Considering the high rates of child cognitive delay in the first 5 years of life in rural China and examining the evidence in this paper in regard to the trajectories of child cognitive development during 0 to 3 years of age in rural China, we recommend that China’s government act to help families to improve the cognitive development of their children at an early age, especially for rural families and those families with low SES. Programs should be established to help families measure levels of child cognitive development when children are young and provide immediate intervention for children with delays, with special consideration for vulnerable communities such as poor households in rural China.
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