The developing brain is exceptionally sensitive to environmental influences, such as chemical agents, nutrition and medical care, as well as to education, experiences and the social environment.1–5 The social environment is composed of familial and cultural characteristics that exist in society and impact on neuropsychological development.6 Early development is recognized as a policy priority since ‘the early childhood period is considered to be the most important developmental phase in an individual's lifespan’.7 A review of proximal risk factors for child development in the context of developing countries identified inadequate cognitive stimulation as the most important psychosocial determinant, along with maternal depression and exposure to violence.8 In developed countries, poor cognitive stimulation at early ages may produce a wide range of non-adaptive neurobehavioral outcomes.9

Early parental influences are crucial in the early stages of brain maturation, and are specifically related to sensitivity and early cognitive stimulation.10 Among these influences, maternal intelligence is an important determinant in the early stages of child neuropsychological development. On average, about 50% of the variance in intelligence is due to genetic differences.11 Apart from the genetic contribution, mothers with higher scores in an intelligence test probably have better parenting skills, involving factors such as dietary patterns, health care, housing, and the provision of a cognitively-stimulating environment. A fundamental characteristic of brain development is that environmental experiences are as important as inherited factors.12 Tong et al.13 found that maternal intelligence quotient (IQ) was positively predictive of children's cognitive development. The same study also found that socioeconomic position and the domestic environment were positively associated with child neuropsychological development. Income, education and occupation have been found to be positively associated with better parenting.14 A cross-cultural review found that socioeconomic indicators were strongly related to cognitive development from infancy to middle childhood.15

In contrast, maternal depression may be considered a risk factor for child development, including impairments in the areas of social-emotional development, behavior, and cognitive development.16–19 The effects of depression on the parenting practices of mothers may include disruption in displays of affection, inadequate attention to basic safety, care and sleep regulation, use of harsh discipline, less utilization of primary care and greater utilization of emergency care.20

The aim of this study was to disentangle the direct effects of maternal IQ and mental health on early childhood neuropsychological development. To reduce residual confounding in the associations, and because of the strong relationship between maternal intelligence and mental health and socioeconomic variables, maternal education and occupational social class were treated as confounders and potential effect modifiers in the analyses. Moreover, a large number of potentially confounding variables, especially those related to child stimulation, were also included.

A population-based birth cohort was established in the city of Sabadell (Barcelona, Catalonia, Spain) as part of the INMA (Environment and Childhood) Project21. Between July 2004 and July 2006, 657 pregnant women who visited the public health center of Sabadell for an ultrasound scan in the first trimester were recruited. These women were then followed-up during pregnancy. A total of 619 (94.2%) children were enrolled at birth, and 588 (89.5%) were followed-up until 14 months. The main analyses in this report are based on 523 children with complete information on maternal IQ-mental health and neuropsychological development assessment. Information on maternal education, socioeconomic background, parity, alcohol consumption, dietary intake, and smoking during pregnancy was obtained through questionnaires administered during pregnancy (first and third trimesters). Anthropometric measures, sex, and gestational age were obtained at birth from clinical records. Data on breastfeeding were based on questionnaires administered to the mothers at 6 and 14 months after delivery. The study was approved by the Clinical Research Ethical Committee of the Municipal Institute of Healthcare (CEIC-IMAS), and all the participating mothers gave informed consent.

The mean age of the children was 14.8 months (range: 12-17 months). Four percent of the children had low birth weight, 2% with a gestational age of less than 37 weeks (preterm), and 5% with a non-optimal score in the Apgar test at 1minute (table 1). Forty-two percent of the mothers had secondary education and 32% had university education. Fifty-three percent were classified as belonging to a non-manual occupational social class. Ten percent of mothers were immigrants and 15% were current smokers during pregnancy. Paternal educational level was similar to that in mothers, but the occupational social class distribution was markedly different. More than 50% of the fathers in the sample were classified as belonging to the manual occupational class. In general, the mothers who participated in the psychological profile tests had higher levels of education than those who did not participate (data not shown). For occupational social class no differences were found.

Distributions of the BSID scores by maternal IQ and mental health, and parental socioeconomic characteristics were plotted (table 2). A positive trend was observed in child mental test score related to maternal intelligence, but not to psychomotor scores. There was a positive trend in the distribution of maternal IQ related to maternal education and occupational social class (table 3). Mothers who had a university degree or who belonged to the non-manual occupational social class scored significantly higher in the IQ test.

The crude associations between parental and child characteristics and mental and psychomotor scales were plotted (fig. 1). In the multivariate regression models, the mental test score significantly increased with higher maternal IQ; after adjusting for all potential confounders except maternal education, the association remained marginally statistically significant (table 4). However, the association disappeared when maternal education was included. No association was observed between the mental test score and maternal mental health. None of the three specific factors of mental health were associated with the mental test score (p >0.10, not shown). No associations were found between the scores of the psychomotor tests and maternal profile variables. Paternal mental health had no effect in the models.

Figure 1.

Bivariate association (coefficient and 95% confidence interval) between outcomes and covariates of interest.

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Finally, these associations were stratified by maternal education and occupational social class (table 5). Within maternal manual occupational social class, there was a significant difference (coef = 7.92) in the mental test score between children whose mothers scored in the highest tertile of maternal IQ compared with children whose mothers scored in the lowest tertile. No such differences were observed, however, among children whose mothers were in the non-manual occupational social class. These differences between strata showed a significant interaction (p for interaction = 0.044). A similar result was found among children whose mothers had primary education (coef = 6.75), but this difference was not statistically significant (p = 0.18), also contrasting with no differences among children whose mothers were highly educated. Maternal mental health showed no difference between different occupational social classes. Maternal education and occupational social class did not modify the relationship between maternal IQ-mental health and infant psychomotor development (data not shown).

In this population-based birth cohort, the effect of maternal intelligence on child cognitive development at 14 months of age was stronger than the effect of maternal mental health; this relationship was found to be mostly explained by maternal education. However, in more disadvantaged occupational social classes, maternal intelligence appeared to be the best predictor of optimal child cognitive development, exceeding both maternal education and mental health.

In previous research,13 an independent association was found between maternal IQ and the cognitive development of toddlers, along with socioeconomic status (SES) and the home environment. In our study, the association observed between maternal IQ and child cognitive development disappeared after adjustment for maternal education, but persisted when maternal occupational social class was included. Some authors have pointed out that general intelligence is highly predictive of educational level.30 This might imply that IQ should exceed educational level, perhaps due to a strong hereditary component, and to some extent should predict educational attainment in life. If this were the case, the scores of children in mental test scores should be strongly associated with maternal IQ and, to a lesser degree, with maternal education. However, our data does not support such a hypothesis. Other authors31 have pointed out that the proportion of variability in a population's IQ that is attributable to genetic factors is unclear. We found that maternal educational attainment was strongly related to child neuropsychological development. Along with SES, low maternal education has been found to be an important predictor of neuropsychological developmental problems in different contexts.10 It has been hypothesized that women with higher IQ scores and higher educational level offer better parenting, involving factors such as lifestyle, healthcare, housing, and the provision of a cognitively-stimulating environment. A relationship between maternal education and dietary patterns in infancy has been found.32 In turn, studies suggest that mothers with low education lack the intellectual skills required to stimulate their children.33

Within manual occupation social classes, maternal IQ appears to be an important predictor of child cognitive development. The children of mothers belonging to a manual occupational social class with high scores in IQ tests scored significantly higher on the BSID mental test than the children of less intelligent mothers belonging to the same occupational social class. Previous studies have found that SES is an important predictor of neuropsychological development, especially for language and executive function.30 There is some support for the hypothesis that less cognitive stimulation is provided by parents and the environment in deprived areas, and that this mechanism may produce disparities in neuropsychological development between different social classes.34 In our study, those mothers belonging to manual occupational social classes but with higher intelligence than the group average represented a special subgroup within the manual class. This finding could indicate that mothers with higher intelligence have children with higher intelligence, irrespective of their educational level. By way of explanation, these mothers might offer their children a more stimulating environment within which they have greater opportunities to develop their cognitive functions.

We found no association between maternal mental health and child neuropsychological development. A possible explanation could be that this study was performed in a non-clinical population in which mothers were not suffering from major depressive disorders or any other serious psychiatric disorders. The distribution of GHQ-12 scores had low variability and did not reveal any subclinical symptomatology. Nevertheless, throughout the school years, cognitive development becomes more specific and, in turn, neuropsychological testing is more precise, reliable and valid than at previous ages.34 The follow-up of the birth cohort will allow us to explore whether an effect on cognitive development appears at older ages, and whether this affects children's social behaviour.

No associations were found between maternal IQ and mental health and child psychomotor development at 14 months. The psychomotor scale showed a stronger association with preterm birth, Apgar scores at 1minute, maternal work status at 14 months, and maternal age. Very preterm infants scored significantly lower than full-term infants on the psychomotor scale of the BSID but no significant differences were found between the groups on the mental test score.35 In other studies, negative Apgar scores have been found to be related to neurological disability.36 Maternal employment at the ninth month was found to be linked to lower scores in a cognitive test at 36 months.37

A major strength of this study is its large sample size. We studied a population-based cohort of 559 children using field staff, interviewers, laboratory technicians, and project pediatricians, all of whom were specifically trained for the project. For the neuropsychological assessments, several quality controls were introduced (inter-observer reliability tests) and the psychologists who assessed children with the BSID questionnaire received extensive training in this questionnaire.

Our study was limited by a number of factors. Although we included a large number of covariates, we were unable to control for some factors that may affect children's cognition, such as the quality of their home environment, nutrition, maternal postpartum depression or maternal stress during pregnancy. Omitting these factors may have introduced residual confounding.38 Moreover, we used self-reported questionnaires for the parents to assess sociodemographic characteristics and mental health, which could have introduced a response bias. However, such a bias would most likely be non-differential and lead to reduced statistical power. Despite the importance of maternal characteristics during the first years of life, information on the father is also valuable. Unfortunately, such information was not available and we could not assess paternal IQ or mental health across the entire sample. Finally, measurements of mental health vary with time. For the assessment of long-term effects, repeated measurements during and just after pregnancy are required.

We conclude that in this general population, variations in maternal mental health were low and their effects on neuropsychological development were not detected at 14 months of age. Maternal IQ plays an important role in the first stages of cognitive development in children in more disadvantaged occupational social classes. For the other groups, the effects of maternal IQ on cognitive development were mostly explained by maternal education. Future research should assess whether these findings continue to be observed at a later stage of development when phenotypes are more highly developed.

J. Forns analyzed the data, drafted the manuscript and gave final approval of the version to be submitted. J. Julvez, M. Vrijheid, M. Guxens, M. Ferrer and J. Sunyer designed the study, developed the questionnaires, supervised the survey and revised the manuscript critically. J. Grellier revised the manuscript critically, contributed to drafting the manuscript, and corrected the quality of the written English. R. Garcia-Esteban contributed to the statistical analyses and read the manuscript critically. All the authors have read and approved the final manuscript.

This work was supported by grants from the Spanish Ministry of Health [FIS-PI041436]; Instituto de Salud Carlos III [Red INMA G03/176 and CB06/02/0041]; the Generalitat de Catalunya-CIRIT [1999SGR 00241]; the EU sixth framework project NEWGENERIS [FP6-2003-Food-3-A-016320]; and Fundació Roger Torné.

None.