Our results show that home environment was the variable with greater influence on neurodevelopment at 18 months. The observation of how parents and children interact in the home context is crucial to adequately evaluate early child development. The development of the central nervous system is affected by the combination of genetic predispositions and social and physical environment.
An adequate brain development depends on both prenatal and early postnatal experiences, and is the result of the continuous interaction between genetics and the context of life [ 1 , 2 ]. In early childhood, one of the main factors affecting child cognitive development is the family environment, which includes both care and stimulation provided to children by the caregivers [ 3 ], and the socioeconomic status SES of the family [ 4 ]. During the first years of life, interaction with caregivers is crucial to ensure adequate psychosocial development of children and the absence of stimulation is associated with early social and cognitive disadvantage [ 5 — 7 ].
The parental socioeconomic status is a multidimensional construct based on several parameters, such as family income, material resources, education and occupation as well as related neighborhood and family characteristics [ 8 ]. The effects of SES on cognitive development of children are well known [ 9 — 11 ]. Children who grow up in families with lower SES are at increased risk of reduced psychological well-being and emotional and cognitive development. SES may affect neural development through a variety of different mediators, such as prenatal factors, parental care, cognitive stimulation, nutrition, stress, toxins and drugs exposure [ 10 ].
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Maternal IQ is also associated with child neurocognitive development. Both a direct and indirect influence of maternal intelligence on child IQ has been suggested, the indirect influence being mediated by family income and home environment [ 12 ]. A recent newborn cohort study showed the independent and positive influence of socioeconomic position, maternal IQ and home environment on child cognitive development [ 1 ] but, so far, only a limited number of studies has evaluated this complex relation providing an adequate and comprehensive adjustment for parental and environmental factors.
In addition, more informative results could derive from the possibility of analyzing separately the different domains of child neurocognitive development cognitive, linguistic and motor. The PHIME project Public health impact of long-term, low-level, mixed element exposure in susceptible population strata , NAC II cohort, had as its main objective, the evaluation of the association between low-level prenatal mercury Hg exposure through maternal fish consumption and child neurodevelopment in a coastal area of the Friuli Venezia Giulia region, in the north east of Italy.
The study protocol and the results on the effects of prenatal Hg exposure have been published elsewhere [ 16 , 17 ]. The enrolment of pregnant women started in April A written informed consent was obtained. At the time of enrolment, on the occasion of ultrasound scans during pregnancy and at birth, biological samples hair, blood, urine, and cord blood were collected and two questionnaires containing information on food habits, socio demographic and health status were filled in by parents. Starting from 18 months after birth, a home visit was planned to evaluate the home environment using the AIRE instrument see below ; finally, in order to evaluate the child neurocognitive abilities at 18 months of age, the Bayley Scales of Infant and Toddler Development Third Edition BSID III, explained in detail below was administered during an ad hoc hospital visit.
Unlike other intelligence tests, SPM is independent from language, reading and writing skills. As a result of these characteristics, it minimizes cultural bias [ 19 ]. This standardized intelligence test evaluates the ability to think clearly and extract meaning out of events; it includes 60 items presented in 5 sets, with 12 items per set. The test consists of geometric analogy problems in which a series of geometric figures is presented with one entry missing, and the correct missing entry must be selected from a set of answer choices.
Raw scores were converted to IQ scores which were used to measure performance compared with norms. The IQ index was then divided into quintiles in order to compare the lowest quintiles with the others. The test was administered by trained psychologists. It includes an interview to the caregivers and an observation of parent-child interaction in the home context in order to evaluate four domains. A communication and affective interaction between parents and child i. The range of possible scores of the AIRE is 0—20 for each subscale.
The AIRE visit lasted 30 to 40 minutes and was conducted by trained psychologists. On the basis of previous studies [ 22 ], these scores were summed for type of employment and educational level the mean between parents was used and the SES index obtained was then divided into quintiles in order to compare the lowest quintiles with the others.
It is an individually administered scale that assesses the development of infants and toddlers between 1 and 42 months of age. The BSID III consists of 5 scales: 1 the cognitive scale includes items on sensorimotor development, exploration and manipulation, object relatedness, concept formation, memory, and other aspects of cognitive processing ; 2 the language scale assesses preverbal behaviors, vocabulary development, vocabulary related to morphological development, understanding of morphological markers, social referencing and verbal comprehension, preverbal communication, vocabulary development, morpho-syntactic development, and includes two subscales evaluating receptive and expressive language ; 3 the motor scale assesses fine motor skill, perceptual motor integration, motor planning, motor speed, the movement of the limbs and torso, static position and dynamic movement, and includes two subscales evaluating fine motor and gross motor ; 4 the social emotional scale; 5 the adaptive behavior scale.
The cognitive, language, and motor scales were administered by trained psychologists, while the social-emotional and adaptive behavior questionnaires were self-administered by the parent or primary caregiver.
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Raw scores were converted to scaled scores. For cognitive scale and for each subscale of language and motor development the normative data had a mean of 10 and a standard deviation of 3. These scores were used to determine the child's performance following the scoring instructions, the total values for language and motor scales were obtained adding the relative subscales values.
For the purpose of this analysis we only used the cognitive, language, and motor scales administered at 18 months of age. Continuous data are expressed as mean and standard deviation, categorical data as absolute frequencies and percentages. The association of the three main exposures and the other potential explanatory variables with the scaled score of the three main scales of the BSID III cognitive, language and motor scale were analyzed with multivariable linear regressions using a stepwise approach. The three main variables were forced into the models, while the potential explanatory variables were retained if the p-value was less than 0.
To avoid loss of statistical power in multivariate analyses, an imputation was carried out for the age of the mother missing in 12 cases , assuming that missing occurred completely at random. For the indirect effects, 5, bootstrap samples were used for bias-corrected bootstrap confidence intervals. Statistical analyses were performed using Stata software One thousand five hundred ninety five women were contacted during pregnancy; three hundred ninety-one resulted not eligible due, for example, to language barriers or insufficient hair length for sampling.
For children it was also possible to carry out the home visit More details on motivations that lead women to withdraw from the study are available elsewhere [ 24 ]. The socio-demographic characteristics of the final sample are presented in Table 1. The enrolled population consists mainly of Italian women with a mean age of Concerning study exposures, the mean value for the standardized IQ score was The mean value obtained for total AIRE was The model including the three exposure variables confirmed the relationship between SES and IQ and the role of AIRE as the main factor affecting child cognitive development.
Multivariate analysis showed that two exposure variables were independently related to language development BSID III language scaled score : maternal IQ, in particular the highest quintile increased the score by 1. The model including the three exposure variables confirmed a relationship between SES and IQ and the role of AIRE as the main factor affecting child cognitive development. No moderation effect of the three main exposures on the study outcomes was found. Our analysis, which was carried out on data collected prospectively in a newborn cohort, confirms the complex relation between home environment, socioeconomic status, maternal IQ and early child neurocognitive development.
In fact, the study results show that the three factors interact with each other to affect the different areas of early child development. Our results show a mediation effect of home environment AIRE "promotion of autonomy" subscale and family SES on the direct relation between maternal IQ a proxy of heritability of intelligence and cognitive and language development: the direct effect of maternal IQ was lost after considering the other two exposures.
Conversely, a direct and indirect effect of maternal IQ on motor development was found. Home environment was the variable with the greatest influence on child neurodevelopment. Our analysis shows that the main factor affecting child cognitive development at 18 months of age is home environment in particular the "promotion of autonomy" subscale. The association between home environment and child cognitive development has already been reported: high scores on the HOME scale the model from which the AIRE scale was derived were associated with more advanced early cognitive development in children [ 25 — 27 ].
A number of studies have addressed the relation between family socioeconomic status and child cognitive development [ 8 — 10 ]. SES may affect neural development through a number of different environmental factors, such as prenatal factors, parental care, cognitive stimulation, nutrition, parental stress, toxins or drugs exposure [ 10 ].
Recently, using magnetic resonance imaging, researchers have revealed a direct effect of SES on children's brain structure. In particular, lower SES was associated with smaller volumes of grey matter in several brain areas. The mediation analysis in this study confirms a direct effect of SES on cognitive score but the effect of home environment is most relevant. The role of maternal IQ on child cognitive development is controversial.
Conflicting evidence have been provided on this issue [ 1 , 12 , 13 , 25 , 30 , 31 ], suggesting both direct and indirect influence of maternal intelligence on child IQ [ 1 , 12 ] but also that this relationship could be influenced by home environment and shared experiences [ 13 , 31 ]. Our results support the latter hypothesis, suggesting a major role of environmental factors and in particular of the home environment.
Our mediation analysis suggests a direct relation between maternal intelligence and child language development and a mediation effect of SES on this relation. The final model indicates that the main factor affecting child language is family environment. Previous studies have shown that SES can affect child language development through family environment.
Hoff found that high-SES children between 12 and 24 month of age grew more, in terms of size of their productive vocabularies, than the mid-SES children. Properties of maternal speech that varied based on SES, fully accounted for this difference [ 32 ]. In particular, in families with higher SES, parents use encouraging sentences more often than they do prohibitive ones, with a supposed effect on child development [ 33 ]. Mediation analysis showed that both home environment and maternal IQ affect child motor development. Also according to previous studies, home environment and parent-child interaction may have a crucial role in the development of motor function [ 34 , 35 ].
The stimulation to manipulate toys and other age-appropriate objects and the encouragement to freely explore the home environment both of which dimensions are associated to the AIRE "promotion of autonomy" subscale result in better motor development, particularly during the early stages of childhood [ 36 ].
Our study shows a direct influence of maternal IQ on motor development. Evidence confirming this direct relationship is scanty [ 37 ]. An indirect effect of maternal intelligence, through a more stimulating child environment, has been shown [ 38 ]. No direct or indirect effect of SES on motor development was found in our study. Evidence on the role of SES on motor development are scarce and conflicting, showing a possible effect of social disadvantage in some cases [ 39 ], and a major role of other factors i. The main strength of the study is the adjustment for a wide range of possible explanatory variables including newborn characteristics sex, birth weight, gestational age at birth, breastfeeding , mother characteristics age of the mother at delivery, BMI before the pregnancy, smoke, alcohol intake , family characteristics mother living with partner, other children living in the house, house size, daycare attendance at 18 months , exposure to heavy metals mercury in blood and hair, dental interventions.
The observation at home of mother-child pair interaction using the AIRE tool is expensive and time consuming. However, our results show that such observation is of paramount importance when studying child development, since the home environment influences the relationship between maternal IQ, SES and child development. In a previous paper we demonstrated the very high interrater reliability of all BSID III scales evaluated, thereby ensuring that comparable results are obtained by the psychologists administering the test [ 42 ].
The use of the latest version of the BSID allowed for separate evaluation of linguistic and cognitive skills thus providing more reliable and informative data on the different developmental areas. The main study limitation is the lack of adjustment for some variables potentially associated with the study outcomes, i. As a result of this, we were unable to fully assess the effects of the genetic component on the neurodevelopmental outcomes.
The transferability of the results may be limited since this study was carried out in a single north eastern Italian city. Furthermore, the "Italian language" inclusion criteria and the loss to follow up of women with low educational level [ 17 ] oversampled Italian highly-educated women with high IQ.
It must be emphasized, however, that the high IQ score could be explained by the Flynn effect, which refers to the observed rise in IQ scores over time resulting in norms becoming obsolete [ 43 ], as indeed is the case for the Italian norms of the Raven test. Finally, the literature suggests that developmental predictors may change over time: i. This effect makes it difficult to draw comparisons between our data and those available in literature, often collected at different ages, using different evaluation tests i.
In the PHIME cohort, the age of 18 months was set by protocol based on the need for early assessment of enrolled children, but several authors have suggested it may be too early to adequately evaluate the neurocognitive development of children [ 45 — 48 ].
Family Environment and Intellectual Functioning: A Life-span Perspective
However, the cohort follow up will continue until the children have reached the age of seven, with an intermediate evaluation at 40 months, allowing for the evaluation of how developmental predictors may have modified in the course of time. Our results show that home environment, socioeconomic status and maternal intelligence are connected with each other in affecting the different domains of early child development. In particular, promotion of autonomy within the home environment seems to play a crucial role, in this age group, in all observed domains cognitive, language and motor.
Therefore, the observation of how parents and children interact in the home context, despite being expensive and time consuming, is essential to adequately evaluate early child development [ 49 ]. This finding may be useful also to plan early interventions aimed at ensuring adequate psychosocial development in children. The European Union is not liable for any use that may be made of the information contained herein, and the study reflects only the views of the authors.
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The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript. National Center for Biotechnology Information , U. PLoS One. Published online May Olga Y Gorlova, Academic Editor. Author information Article notes Copyright and License information Disclaimer. Competing Interests: The authors have declared that no competing interests exist. Received Aug 7; Accepted Apr This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are properly credited.
This article has been cited by other articles in PMC. S2 File: Results of mediation analysis for child language development.
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