Abstract

Objective

Assessing the association between socioeconomic gradient and cognitive development among children of a Spanish birth cohort aged 5-6 years from a gender perspective.

Method

Cognitive development was assessed on 525 children aged 5-6 years in the INMA-Valencia cohort, with the Global Cognitive Score (GCS) from McCarthy Scales of Children's Abilities. Information on social class, education level and employment was collected for both parents in addition to other sociodemographic factors, parental, family and child characteristics. The relationship between maternal and paternal socioeconomic gradient and cognitive development was assessed by linear regressions and comparing the variance explained by each indicator measured in the mother and father.

Results

Maternal socioeconomic gradient indicators explained more variance on GCS than paternal. Maternal education and paternal social class had an important individual effect that stayed after adjusting by other parental, child and family determinants. In the multivariable analysis, maternal education, age and intelligence, paternal social class and the child's age and sex were significantly associated with cognitive development.

Conclusions

Diverse socioeconomic gradient factors have an important influence on cognitive development, maternal education being the strongest determinant. Policies should be implemented to mitigate the negative effects of this gradient on child development.

Keywords:
Cognitive development; Socio-economic gradient; Children; Parental education; Gender perspective

Resumen

Objetivo

Evaluar la asociación del gradiente socioeconómico y el desarrollo cognitivo en niños y niñas de una cohorte española a los 5-6 años de edad desde una perspectiva de género.

Método

Se evaluó el desarrollo cognitivo en 525 niños/as de 5-6 años de la cohorte INMA-Valencia, mediante la Puntuación Global Cognitiva (PGC) de las Escalas McCarthy para niños y niñas. Se recogió información de ambos progenitores sobre clase social, nivel de estudios y empleo, además de otros factores sociodemográficos, características parentales, de la familia y del niño o la niña. La relación entre el gradiente socioeconómico materno y paterno y el desarrollo cognitivo se evaluó mediante modelos de regresión lineal y comparando la varianza explicada por cada uno de los indicadores medidos en la madre y en el padre.

Resultados

Los indicadores de gradiente socioeconómico de la madre explicaron más varianza del índice de PGC que los del padre. La educación materna y la clase social paterna tuvieron un importante efecto individual, que se mantuvo tras ajustar por otros determinantes de los progenitores, del niño o de la niña, y del entorno familiar. En el análisis multivariante, la educación, la edad y la inteligencia maternas, la clase social paterna, y la edad y el sexo del infante se asociaron significativamente con el desarrollo cognitivo.

Conclusiones

Distintos factores del gradiente socioeconómico tienen influencia en el desarrollo cognitivo, siendo la educación materna el determinante más fuerte. Deberían implementarse políticas para paliar los efectos negativos de este gradiente en el desarrollo infantil.

Palabras clave:
Desarrollo cognitivo; Gradiente socioeconómico; Infancia; Educación parental; Perspectiva de género

Introduction

Cognitive development in early childhood is crucial: it has been considered the most important period of development during a person's lifespan,¹1. Human Early Learning Partnership, Commission on Social Determinants of Health. Early child development: a powerful equalizer: final report for the World Health Organization's Commission on the Social Determinants of Health. Prepared by Arjumand Siddiqi, Lori G. Irwin, Dr. Clyde Hertzman. 2007. Vancouver: Human Early Learning Partnership. Available at: http://www.who.int/iris/handle/10665/69729
http://www.who.int/iris/handle/10665/697... and it influences adaptation skills and predicts academic performance and educational attainment.

Health inequalities are highly avoidable differences (based on personal, demographic or social characteristics) among diverse population groups. One of the most described factor producing health inequalities is socio-economic position in the family context: it could determine child's physical and cognitive development²2. Flores M, García-Gómez P, Zunzunegui M-V. Crisis económica, pobreza e infancia. ¿Qué podemos esperar en el corto y largo plazo para los "niños y niñas de la crisis"? Informe SESPAS 2014. Gac Sanit. 2014;28 Suppl 1:132-6.. Socio-economic gradient refers to the classification of individuals along a gradation of economic, working or educational attributes such as income, occupation or education. Socioeconomic determinants of health find different pathways to express influence on cognitive development. Households with restricted economic resources could have worse conditions: low investment on offspring's education, poor housing, and living in neighbourhoods with less community services.³3. Shaw DS, Shelleby EC. Early-starting conduct problems: intersection of conduct problems and poverty. Annu Rev Clin Psychol. 2014;10:503-28.^-44. Goldfeld S, Woolcock G, Katz I, et al. Neighbourhood effects influencing early childhood development: conceptual model and trial measurement methodologies from the kids in communities study. Soc Indic Res. 2015;120:197-212. Parental working status could trigger parental stress, and more stressed parents usually display more problematic interactions with their children.⁵5. Khanam R, Nghiem S. Family income and child cognitive and noncognitive development in Australia: does money matter? Demography. 2016;53:597-621. Parental education level reflect parents' personal resources and problem-solving abilities.³3. Shaw DS, Shelleby EC. Early-starting conduct problems: intersection of conduct problems and poverty. Annu Rev Clin Psychol. 2014;10:503-28. These three determinants (income, working status, and education) generate a unique cognitive stimulation that affect children's cognitive development.⁶6. Shavers VL. Measurement of socioeconomic status in health disparities research. J Natl Med Assoc. 2007;99:1013. Therefore, assessing different indicators of socio-economic gradient and their relationship to child's cognitive development could be a more comprehensive approach.

In addition to this, a gender perspective should be considered when assessing the differences in social class,⁷7. Borrell C, Artazcoz L. Las desigualdades de género en salud: retos para el futuro. Rev Esp Salud Pública. 2008;82:241-9. employment and time use⁸8. Hormiga-Sánchez CM, Alzate-Posada ML, Borrell C, et al. Actividad física ocupacional, de transporte y de tiempo libre: desigualdades según género en Santander, Colombia. Rev Salud Pública. 2016;18:201-13. of mothers and fathers according to gender roles. Due to gender inequalities, the effect of socio-economic gradient indicators on offspring development is likely to be different for fathers and mothers.⁹9. Erola J, Jalonen S, Lehti H. Parental education, class and income over early life course and children's achievement. Res Soc Stratif Mobil. 2016;44:33-43. It is suggested that fathers usually provide material assets while mothers' endowments are based on their own academic achievement.⁹9. Erola J, Jalonen S, Lehti H. Parental education, class and income over early life course and children's achievement. Res Soc Stratif Mobil. 2016;44:33-43. Additionally, it seems that education play different roles: mother's education is relevant for academic achievement at child's early years, while father's is more important at youth.⁹9. Erola J, Jalonen S, Lehti H. Parental education, class and income over early life course and children's achievement. Res Soc Stratif Mobil. 2016;44:33-43.^-1010. De Pablos Escobar L, Gil Izquierdo M. Intergenerational educational and occupational mobility in Spain: does gender matter? Br J Sociol Educ. 2016;37:721-42. However, measurement of these characteristics could be affected by gender bias: usually, due to heteropatriarchal values, paternal socioeconomic position has been considered, as a more stable way to measure familiar social class (traditionally, men have spent more time in labour market than women).⁹9. Erola J, Jalonen S, Lehti H. Parental education, class and income over early life course and children's achievement. Res Soc Stratif Mobil. 2016;44:33-43.^,1111. Borrell C, Rohlfs I, Artazcoz L, et al. Desigualdades en salud según la clase social en las mujeres. ¿Cómo influye el tipo de medida de la clase social? Gac Sanit. 2004;18 Supl 2:75-82. For this reason, a gender perspective could help to visualize the differential pattern of mother's and father's socio-economic gradient indicators within the family and to compare their relative contribution to child's development.

Furthermore, cognitive development may be influenced by factors that can mitigate or interact with the effect of social disadvantage. Some factors commonly studied in association with child cognitive development are: parental age, family structure, immigrant condition,¹²12. Chen C-J, Hsu C-W, Chu Y-R, et al. Developmental status and home environment among children born to immigrant women married to Taiwanese men. Res Nurs Health. 2012;35:121-31.

13. Moura DR, de Costa JC, Santos IS, et al. Risk factors for suspected developmental delay at age 2 years in a Brazilian birth cohort. Paediatr Perinat Epidemiol. 2010;24:211-21.

14. Edwards RD, Roff J. Negative effects of paternal age on children's neurocognitive outcomes can be explained by maternal education and number of siblings. PloS One. 2010;5:e12157.^-1515. Rausch JC, McCord M, Batista M, et al. Latino immigrant children's health: effects of sociodemographic variables and of a preventive intervention program. Int J Popul Res. 2012;2012:1-8. maternal intelligence, mental health, and lifestyles,¹⁶16. Forns J, Julvez J, García-Esteban R, et al. Maternal intelligence-mental health and child neuropsychological development at age 14 months. Gac Sanit. 2012;26:397-404.^,1717. Kiernan KE, Huerta MC. Economic deprivation, maternal depression, parenting and children's cognitive and emotional development in early childhood. Br J Sociol. 2008;59:783-806. child's sex and perinatal outcomes,¹³13. Moura DR, de Costa JC, Santos IS, et al. Risk factors for suspected developmental delay at age 2 years in a Brazilian birth cohort. Paediatr Perinat Epidemiol. 2010;24:211-21.^,1818. Dilworth-Bart JE, Poehlmann JA, Miller KE, et al. Do mothers' play behaviors moderate the associations between socioeconomic status and 24-month neurocognitive outcomes of toddlers born preterm or with low birth weight? J Pediatr Psychol. 2011;36:289-300. caregiving, parenting stress, and practices and home environment (cognitive stimulation).¹⁷17. Kiernan KE, Huerta MC. Economic deprivation, maternal depression, parenting and children's cognitive and emotional development in early childhood. Br J Sociol. 2008;59:783-806. However, few studies have considered a wide range of these factors when assessing the relationship between socio-economic gradient and child cognitive development.

Spanish children are at increased risk of poverty since the start of the economic crisis (2008),¹⁹19. Karanikolos M, Mladovsky P, Cylus J, et al. Financial crisis, austerity, and health in Europe. Lancet. 2013;381:1323-31. and at a greater risk of worse health.²⁰20. Rajmil L, Artazcoz L, García-Gómez P, et al. Spanish children are most affected by the economic crisis, says the Spanish Society of Public Health (SESPAS). BMJ. 2015;350:h1945. In Spain, 33.4% of children are at risk of poverty and social exclusion,²¹21. European Anti Poverty Network (Internet). 6º INFORME 2016: El estado de la pobreza: seguimiento del indicador de riesgo de pobreza y exclusión social en España 2009-2015. 2016 (cited 24 March 2018). Available from: https://www.eapn.es/estadodepobreza/ARCHIVO/documentos/InformeAROPE2016.pdf.
https://www.eapn.es/estadodepobreza/ARCH... this reinforces the need of assessing the relationship between socio-economic gradient and child cognitive development. Moreover, the high unemployment rate in Spain over the last years,²²22. Eurostat [Internet]. Your key to European statistics. Employment rate by sex, age group 20-64. 2018 (cited 29 September 2014). Available from: https://ec.europa.eu/eurostat/statistics-explained/index.php/Unemploymentstatistics.
https://ec.europa.eu/eurostat/statistics... justifies the exploration of employment situation as an additional indicator of socio-economic gradient and its relationship with child cognitive development. However, recent evidence on the effects of socio-economic gradient on cognitive development of preschool children in Spain is scarce and relies mainly on occupational social class as an indicator of socio-economic position.¹³13. Moura DR, de Costa JC, Santos IS, et al. Risk factors for suspected developmental delay at age 2 years in a Brazilian birth cohort. Paediatr Perinat Epidemiol. 2010;24:211-21.^,1616. Forns J, Julvez J, García-Esteban R, et al. Maternal intelligence-mental health and child neuropsychological development at age 14 months. Gac Sanit. 2012;26:397-404.^-1717. Kiernan KE, Huerta MC. Economic deprivation, maternal depression, parenting and children's cognitive and emotional development in early childhood. Br J Sociol. 2008;59:783-806. Additionally, the exploration of parental characteristics from a gender approach seems basic to understand family dynamics deeply.¹⁰10. De Pablos Escobar L, Gil Izquierdo M. Intergenerational educational and occupational mobility in Spain: does gender matter? Br J Sociol Educ. 2016;37:721-42. As far as we know, this is the first work to assess child cognitive development from a parental socioeconomic and gender perspective.

The aim of the present study is to compare the contribution of maternal and paternal occupational social class, education and employment situation to cognitive development of children of the INMA (INfancia y Medio Ambiente Environment and Childhood) cohort in Valencia (Spain) at age 5-6 years, taking into account other socio-demographic, socio-familial, parental, and child factors.

Method

Study design and population

INMA Project is a Spanish population-based mother-and-child multicenter cohort set up in 2003 and composed by seven cohorts (Ribera d'Ebre, Granada, Menorca, Valencia, Sabadell, Asturias, and Gipuzkoa). This study uses data from INMA-Valencia cohort. Recruitment process and subsequent procedures are described in more detail elsewhere.²³23. Guxens M, Ballester F, Espada M, et al. Cohort profile: the INMA -INfancia y Medio Ambiente-(Environment and Childhood) Project. Int J Epidemiol. 2012;41:930-40. Briefly, women were recruited during their first prenatal visit to their reference hospital (La Fe, in Valencia) before week 13 of gestation by consecutive sampling of those who met the inclusion criteria ( 16 years old, singleton pregnancy, non-assisted conception, delivery at the reference hospital, and no communication handicap). Out of 1578 eligible women, 855 accepted to participate and were included in the study between November 2003 and June 2005. Participants were more likely to be employed and slightly older than non-participants. Cognitive development was assessed in 525 children aged 5.5-6.5. The flow chart of the INMA participants included in the study is shown in Figure 1.

All participating families gave their written informed consent. This study was approved by the Ethics Committee of La Fe Hospital and conforms to the principles embodied in the Declaration of Helsinki.

Cognitive development assessment

Child cognitive development was assessed using a Spanish adapted version of the McCarthy Scales of Children's Abilities (MSCA).²⁴24. McCarthy D. MSCA: escalas McCarthy de aptitudes y psicomotricidad para niños. Manual. 8.a ed. Madrid: TEA; 2006. Available at: http://data.theeuropeanlibrary.org/BibliographicResource/3000146010697
http://data.theeuropeanlibrary.org/Bibli... Two trained psychologists administered and interpreted the MSCA, following a strict protocol to avoid inter-observer variability. Scale alpha coefficients were >0.70, except for MSCA motor subscale 0.64 (good to moderate). The interrater reliability was estimated by intraclass correlation with coefficients >0.77.²⁵25. Valera-Gran D, Navarrete-Muñoz EM, García de la Hera M, et al. Effect of maternal high dosages of folic acid supplements on neurocognitive development in children at 4-5 y of age: the prospective birth cohort Infancia y Medio Ambiente (INMA) study. Am J Clin Nutr. 2017;106:878-87. The MCSA contains eighteen subtests grouped into three global sub-scales (verbal, perceptive-performance, numeric). The General Cognitive Score (GCS) is obtained by summing up their direct punctuation. Raw GCS, adjusted for child sex and age at evaluation, were used in the analyses.

Information on socio-economic gradient and other variables were collected applying structured questionnaires by experienced interviewers. Information was collected independently to family structure, by obtaining it from the parent who brought the child to the follow-up visit or by telephone contact.

Socio-economic gradient variables

Socio-economic gradient was measured for mothers and fathers through three variables: occupational social class, widely used in Spain as a measure of socioeconomic position²⁶26. Working Group of the Spanish Society of Epidemiology and the Spanish Society of Family and Community Medicine, Domingo-Salvany A, Regidor E, Alonso J, et al. Proposal for a social class measure. Aten Primaria. 2000;25:350. it was defined using a Spanish adaptation of the British social class classification. In this study, we used the abbreviated version, with five classes, recoded in three categories: higher (I+II), medium (III) and lower (IV+V). Social class and educational level (primary or less, secondary or university) were requested during pregnancy. Employment situation (employed, unemployed and homemaker or student) was collected at 4 and 5 years of age, and was recombined reflecting stability or instability at both time points as follows: employed at 4 and 5, homemaker or student at 4 and 5, employed at 4 or 5, and unemployed at 4 and 5. When homemaker or student positions were combined at one time point with employed or unemployed status in the other time point, cases were reassigned as employed or unemployed.

Other variables

Statistical analysis

Basal linear regression models adjusted for child's sex and age were fitted to evaluate the association between child's GCS and the study variables, in particular, the differential role of maternal and paternal characteristics was evaluated and graphically represented. Subsequently, the proportion of residual variance explained by maternal and paternal socioeconomic characteristics (education, social class and employment status) was assessed after adjusting a basal model including only the control variables: child's sex, age at evaluation, and maternal age. By means of sequential models, we decomposed the explained variance into independent or direct effects and shared or common effects. The parts of the shared effect that were related to each combination of the three variables was represented with the aid of Venn diagrams by means of the “eulerr” package of R.³⁰30. Larsson J, Godfrey AJR, Kelley T, et al. EULERR: Area-proportional Euler and Venn diagrams with circles or ellipses. version 4.1.0. (Accessed 27/6/2018.) Available at: https://rdrr.io/cran/eulerr/
https://rdrr.io/cran/eulerr...

Multivariate linear models were also built to determine the relationship of socio-economic gradient indicators with the child GCS, adjusting for the other potential determinants. The first model (1) included sociodemographic factors and socio-economic gradient variables excluding sequentially those variables not related at p <0.10 in the adjusted model following a backward stepwise selection procedure. The subsequent models included the variables retained in model (1) and all the variables considered in each block: (2) parental psychological characteristics; (3) variables related to the socio-familial environment; (4) Pregnancy exposure to toxic substances; and (5) child's characteristics. A complete final model (6) was also fitted considering the variables from all groups (1-5) and excluding sequentially those variables not related at p<0.10 following a backward stepwise selection procedure. All models were adjusted by child's sex and age. Residual analysis was conducted to check for model assumptions. Effect modification between maternal and paternal occupational social class, education, and employment situation was finally evaluated by including both variables in the models and an interaction term between them. In order to avoid overfitting, these additional models were not simultaneously adjusted for other socio-economic gradient variables. Statistical analysis was carried out using the statistical software R, version 3.4.0.

Results

Descriptive analyses are shown in Table 1. The mean (SD) age of children was 5.8 (0.1) years (range: 5.5-6.4), 51.4% were boys and 48.6% were girls. Concerning socio-economic gradient indicators, 54.5% of mothers and 63.9% of fathers belonged to the lowest social class, respectively. Regarding employment situation, 67.7% of mothers and 81.9% of fathers were employed at 4 and at 5 years, and 10.7% of mothers and 5.4% of fathers were long-term unemployed. Mothers showed a higher educational level as compared to fathers (about 72.4% of mothers completed secondary or university education vs. 55.8% of fathers). Mean GCS was 174 (SD: 19.4; range: 90-229).

Comparative regressions adjusted by child's sex and age are shown for mother and father in Figure 2. A strong gradient between social class and GCS was shown for both mothers and fathers, being stronger for fathers. Regarding employment situation, mothers who were either unemployed at 4 or 5 years of age had children with a 6.8 points lower GCS than those with a sustained employment. For fathers, the increased risk for their children was when they were long-term unemployed (1 = 7.8). Educational level for both mother and father had a strong relationship with child's GCS, being stronger for mothers. Considering the rest of variables in Figure 2, intelligence of mothers and fathers were positively related to child's GCS, although stronger and statistically significant only in mothers (1 = 5.5). Maternal time devoted to household tasks (1 = 3.7) was inversely associated with the GCS, while no relationship was observed with the time spent by fathers.

The association between child's GCS and other general characteristics (family organization, maternal exposure to toxicants in pregnancy, and child's characteristics) are shown in Figure 3. Only smoking in the first trimester of pregnancy (1 = 5.4) and being a boy (1 = 9.1) were found to have an association.

Individual and shared effects for maternal and paternal socioeconomic gradient indicators are represented in Figure 4. The mother was the biggest individual contributor to socio-economic gradient effect on child's GCS with a 5.2% of the variance independently explained, while father accounted for only 1.9%. Shared contributions to socio-economic gradient influence on GCS reached 3.5%, leaving a residual variance of 80.2%.

Considering those variables composing socio-economic gradient, education is the biggest individual contributor to maternal socio-economic gradient (4.4% of variance explained with direct effects and 3.6% with shared effects), while the same was true for paternal social class (1.6% and 2.5%). In both cases, the variance directly explained by employment situation was similar and of low magnitude (0.5% and 0.4%, respectively).

Figure 5 represents the sequential models exploring predictors of cognitive development. Paternal intelligence and mental health were excluded due to missing data. Model 1 shows the relationship between the socio-economic gradient and other sociodemographic factors and child GCS. Paternal occupational social class as well as maternal age and education were significantly related to child GCS. Sequential adjustment for parental psychological characteristics (model 2), socio-familial environment (model 3), pregnancy use of toxic substances (model 4) and child's characteristics (model did not change substantially the magnitude of these estimates, in fact they remained very stable along the subsequent analyses. In the final model adjusted by child's age and sex, only paternal social class, maternal education, age and intelligence, and being small for gestational age were associated.

Discussion

The present study assessed socio-economic gradient (parental social class, education and working status) and its relationship to child cognitive development from a gender perspective, considering other additional relevant factors existing in the children's environment. Comparative analyses were performed between maternal and paternal characteristics to estimate their common and individual weight on child's cognitive development. The observed trends on socio-economic gradient variables have shown lower scores on cognitive development at lower social class and education in both mothers and fathers. Regarding employment situation, lower cognitive development scores were found for those children whose mothers were unemployed at 4 or 5 years of age, while the same was true for those children whose fathers were unemployed at both time points. In order to compare the relative contribution of socio-economic gradient indicators, we decomposed the variance of child cognitive development according to social class, education and working status of each parent. The results suggest that the mother was bigger individual contributor than the father to socio-economic gradient effect on child's GCS. Furthermore, education matters most in the maternal socioeconomic gradient, while the same was true for social class in the paternal socio-economic gradient. Employment situation had lower impact in both cases. After adjusting for other potential determinants, the same pattern was observed: maternal educational level and paternal social class were found to play the most important role on of the social gradient of child cognitive development.

Firstly, regarding parental education, few studies considered separately maternal and paternal schooling. Some found that maternal but not paternal education had an important effect on infant neurodevelopment;³¹31. Koutra K, Chatzi L, Roumeliotaki T, et al. Socio-demographic determinants of infant neurodevelopment at 18 months of age: Mother-Child Cohort (Rhea Study) in Crete Greece. Infant Behav Dev. 2012;35: 48-59.^-3232. Camargo-Figuera FA, Barros AJD, Santos IS, et al. Early life determinants of low IQ at age 6 in children from the 2004 Pelotas Birth Cohort: a predictive approach. BMC Pediatr. 2014;14:308. however, in one of them data on paternal schooling was not available.³²32. Camargo-Figuera FA, Barros AJD, Santos IS, et al. Early life determinants of low IQ at age 6 in children from the 2004 Pelotas Birth Cohort: a predictive approach. BMC Pediatr. 2014;14:308. A study comprising four low-income countries³³33. Crookston BT, Forste R, McClellan C, et al. Factors associated with cognitive achievement in late childhood and adolescence: the Young Lives cohort study of children in Ethiopia, India Peru, and Vietnam. BMC Pediatr. 2014; 14:253. found association with the education of both parents, but in most of them, mothers' schooling had the strongest influence. In our study population there is a strong association in the unadjusted basic models with both maternal and paternal education, but with a higher influence of maternal education. This is in line with other studies,²⁸28. Axelrod BN. Validity of the Wechsler Abbreviated Scale of Intelligence and other very short forms of estimating intellectual functioning. Assessment. 2002;9:17-23.

29. Derogatis LR. SCL-90-R. Madrid: TEA; 2002.^-3030. Larsson J, Godfrey AJR, Kelley T, et al. EULERR: Area-proportional Euler and Venn diagrams with circles or ellipses. version 4.1.0. (Accessed 27/6/2018.) Available at: https://rdrr.io/cran/eulerr/
https://rdrr.io/cran/eulerr... and reinforces the theory which defends that maternal rather than paternal education seems more influential to child's cognitive development at young ages.⁹9. Erola J, Jalonen S, Lehti H. Parental education, class and income over early life course and children's achievement. Res Soc Stratif Mobil. 2016;44:33-43. Women usually tend to assume gender roles with reproductive and domestic unpaid tasks.³⁴34. Sánchez-López MP, Limiñana-Gras RM. Health from a gender perspective: the state of the art. In: The psychology of gender and health. San Diego: Academic Press; 2017. p. 1-52. (Accessed 13/6/2018.) Available at: https://www.sciencedirect.com/science/article/pii/B9780128038642000018 Reconciling this gender role with an increasing presence in labour market is often complicated, and usually women have to cope with both roles in paid (labour market) and unpaid (homemaker) work.³⁵35. Forste R, Fox K. Household labor, gender roles, and family satisfaction: a cross-national comparison. Journal of Comparative Family Studies. 2012;43: 613-31. For this reason, usually women decide to stay outside the labour market during early years of their offspring, offering them a permanent cognitive stimulation.¹¹11. Borrell C, Rohlfs I, Artazcoz L, et al. Desigualdades en salud según la clase social en las mujeres. ¿Cómo influye el tipo de medida de la clase social? Gac Sanit. 2004;18 Supl 2:75-82. This fact could explain that maternal education has a stronger weight on child's neurodevelopment.⁹9. Erola J, Jalonen S, Lehti H. Parental education, class and income over early life course and children's achievement. Res Soc Stratif Mobil. 2016;44:33-43.^,3131. Koutra K, Chatzi L, Roumeliotaki T, et al. Socio-demographic determinants of infant neurodevelopment at 18 months of age: Mother-Child Cohort (Rhea Study) in Crete Greece. Infant Behav Dev. 2012;35: 48-59.^,3333. Crookston BT, Forste R, McClellan C, et al. Factors associated with cognitive achievement in late childhood and adolescence: the Young Lives cohort study of children in Ethiopia, India Peru, and Vietnam. BMC Pediatr. 2014; 14:253.

Regarding occupational social class, a meta-analysis using composite measures of family socio-economic gradient³⁶36. Letourneau NL, Duffett-Leger L, Levac L, et al. Socioeconomic status and child development. J Emot Behav Disord. 2013;21:211-24. reported a significant relationship with child cognitive development, however, several important confounders were not considered. In our study, after adjusting for a wide range of potential confounders, paternal occupational social class was positively related to child's cognitive development. Although maternal occupational social class did not enter the adjusted models, this could be due to its high correlation with maternal education. Additionally, we employed an occupation-based indicator of social class measured in pregnancy, considering the longest employment in the previous nine months.³⁷37. González-Galarzo MC, García AM, Estarlich M, et al. Prevalencias de exposición a riesgos laborales en trabajadoras embarazadas (proyecto INMA-Valencia). Gac Sanit. 2009;23:420-6. In our sample, we have observed high rates on job instability in women, who tend to accept more precarious jobs.⁷7. Borrell C, Artazcoz L. Las desigualdades de género en salud: retos para el futuro. Rev Esp Salud Pública. 2008;82:241-9. Our data could suggest that women's reinsertion in labour market could eventually provoke possible changes in social class that have been unmeasured. High rate of father's employment in comparison to mother's,⁹9. Erola J, Jalonen S, Lehti H. Parental education, class and income over early life course and children's achievement. Res Soc Stratif Mobil. 2016;44:33-43. could supply a more accurate measure to assess its effect on child's cognitive development. Indicators of socio-economic gradient such as occupation or education do not determine simply the household's income, they could be also defining economic environment at neighbourhood level,⁴4. Goldfeld S, Woolcock G, Katz I, et al. Neighbourhood effects influencing early childhood development: conceptual model and trial measurement methodologies from the kids in communities study. Soc Indic Res. 2015;120:197-212. psychosocial stimulation,³⁶36. Letourneau NL, Duffett-Leger L, Levac L, et al. Socioeconomic status and child development. J Emot Behav Disord. 2013;21:211-24. home environment or routines,³²32. Camargo-Figuera FA, Barros AJD, Santos IS, et al. Early life determinants of low IQ at age 6 in children from the 2004 Pelotas Birth Cohort: a predictive approach. BMC Pediatr. 2014;14:308. and quality of parenting, which are related to cognitive development.

Employment situation in both parents had a relatively low contribution to cognitive development variability and did not stay as a predictor in our final model. However, its comparative analysis deserves our attention. Maternal employment situation showed that children with lower cognitive development were those whose mothers had been unemployed at 4 or 5 years of age. Women but not men were at a significative risk of having mental health problems when they are in a temporary job,³⁸38. Pirani E, Salvini S. Is temporary employment damaging to health? A longitudinal study on Italian workers. Soc Sci Med. 2015;124:121-31. this could be damaging mother-child communication, and affecting child's cognitive development.⁵5. Khanam R, Nghiem S. Family income and child cognitive and noncognitive development in Australia: does money matter? Demography. 2016;53:597-621. Child's cognitive development was not affected very deeply by temporary jobs in fathers: in fact, lowest scores on cognitive development were registered in those children whose fathers were long-term unemployed. It could be thought that these fathers could be using their time in unpaid work at home; however, additional analyses of our data have not shown differences of paternal time devoted to domestic work according to working situation. A study described this behaviour by stating that unemployed men have a decline in their well-being and rather than employing more time in house tasks, they tend to make less unpaid work than their employed counterparts.³⁹39. Van der Lippe T, Treas J, Norbutas L. Unemployment and the division of housework in Europe. Work Employ Soc. 2017, 0950017017690495.
https://doi.org/0950017017690495...

This study has several strengths: firstly, we considered different socio-economic gradient variables (social class, education, and employment disaggregated by mother and father), which give a richer representation of socio-economic gradient. Secondly, a wide range of variables describing the children's environment and its potential influence on cognitive development were also considered. Thirdly, its prospective nature allows the use of data collected at different stages enabling the study of the long-term effects of diverse factors measured since the beginning of life. Finally, the cognitive development was assessed by trained psychologists and using a widely recognised and validated instrument.

One of the limitations of this study lies in the fact that some factors potentially influencing on child cognitive development have been unmeasured, e.g. parenting conditions, family routines, psychosocial stimulation, or community resources. Moreover, several variables were available but insufficient for the father, such as paternal intelligence or paternal mental health, and could not be included in multivariate analysis. Future designs must consider information of both parents for the whole sample. Finally, our results might not be generalizable to other settings due to sample attrition. Participant families could have special characteristics, such as being more concerned about cognitive development or having higher socio-economic gradient. In fact, participant families at age five had higher social class and educational level than nonparticipant, and these differences were evident both in the case of men and women (data not shown). Despite this, we observed a social gradient, which might be deeper in the general population.

Knowledge about the magnitude of the effects of socioeconomic gradient on cognitive development and its differential pattern from a gender perspective could help planning interventions aimed to mitigating the negative impact of disadvantaged socioeconomic conditions on child development. Implementation of income equalization to avoid uneven wealth distribution, community programs to support disadvantaged families, and ensuring a public and free access to high education could optimize offspring's cognitive development. Since women appears to have a greater impact on social gradient of cognitive development, public policy interventions aimed to reduce social gender inequalities should be reinforced to improve womens' health and child development.

Future analysis will be performed in order to disentangle the impact of socio-economic gradient (including poverty and social exclusion indicators) on cognitive development and the presence of internalizing and externalizing problems in our cohort. Additionally, family dynamics and home organization will be represented to assess their mediating roles.

In conclusion, indicators of socio-economic condition (parental education and social class) have a differential effect on child's cognitive development, being the mother the greater contributor at the age of 5-6 years in Spanish children from the general population. Findings might be used to implement policies that mitigate the impact of adverse socio-economic gradient on child cognitive development.

Acknowledge

The authors would particularly like to thank all participants for their generous collaboration; and Amparo Cases and Mar González for their support with the fieldwork. A full list of the INMA Project researches can be found at http://www.proyectoinma.org.

References

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