Copenhagen Perinatal Cohort Research Articles

Early Motor Developmental Milestones and Personality Traits in Midlife: A 50-Year Follow-Up Study

Background The purpose of this study was to investigate if infants' age at attaining motor developmental milestones is associated with the big five personality traits 50 years later. Methods Mothers of 8395 infants from the Copenhagen Perinatal Cohort recorded a total of 12 motor developmental milestones during the first year of their infant's life. Information on at least one milestone was available for 1307 singletons with adult follow-up scores on the NEO-Five-Factor Inventory. The mean age at personality testing was 50.1 years. Results Slower attainment of motor milestones was associated with increased neuroticism and lower conscientiousness in midlife. All 12 motor developmental milestones explained a total of 2.4% of the variance in neuroticism, while they explained 3.2% of the variance in conscientiousness. These results remained significant after adjustment for the included family and perinatal covariates, as well as adult intelligence. Discussion The personality trait of neuroticism is a general risk factor for psychopathology and has in young adulthood been found to be associated with early motor development. However, evidence on associations of motor developmental milestones with other personality traits has been non-existent. These findings suggest that delays in early motor development may not only characterise individuals with later psychopathology, including schizophrenia, but may also be associated with personality traits such as neuroticism and conscientiousness through the life course.

Midlife perceived stress is associated with cognitive decline across three decades

BackgroundResearch indicates detrimental effects of stress on brain health and cognitive functioning, but population-based studies using comprehensive measures of cognitive decline is lacking. The present study examined the association of midlife perceived stress with cognitive decline from young adulthood to late midlife, controlling for early life circumstances, education and trait stress (neuroticism).MethodsThe sample consisted of 292 members of the Copenhagen Perinatal Cohort (1959–1961) with continued participation in two subsequent follow-up studies. Cognitive ability was assessed in young adulthood (mean age 27 years) and midlife (mean age 56 years) using the full Wechsler Adult Intelligence Scale (WAIS), and perceived stress was measured at midlife using the Perceived Stress Scale. The association of midlife perceived stress with decline in Verbal, Performance and Full-Scale IQ was assessed in multiple regression models based on Full Information Maximum Likelihood estimation.ResultsOver a mean retest interval of 29 years, average decline in IQ score was 2.42 (SD 7.98) in Verbal IQ and 8.87 (SD 9.37) in Performance IQ. Mean decline in Full-scale IQ was 5.63 (SD 7.48), with a retest correlation of 0.83. Controlling for parental socio-economic position, education and young adult IQ, higher perceived stress at midlife was significantly associated with greater decline in Verbal (β = − 0.012), Performance (β = − 0.025), and Full-scale IQ (β = − 0.021), all p < .05. Across IQ scales, additionally controlling for neuroticism in young adulthood and change in neuroticism had only minor effects on the association of midlife perceived stress with decline.ConclusionsDespite very high retest correlations, decline was observed on all WAIS IQ scales. In fully adjusted models, higher midlife perceived stress was associated with greater decline on all scales, indicating a negative association of stress with cognitive ability. The association was strongest for Performance and Full-scale IQ, perhaps reflecting the greater decline on these IQ scales compared to Verbal IQ.

Experience of sexual violence and satisfaction with life: a 20-year prospective cohort study

ABSTRACT This Danish prospective cohort study assessed associations between exposure to any kind of sexual violence (SV), exposure to labelled SV (self-labelled as rape, sexual abuse, or incest), and satisfaction with life (SWL) in young adulthood (approximately 32 years) (N = 688) and at midlife (up-to-20 years later) (n = 230). Data are from the Prenatal Development Project (a subsample of the Copenhagen Perinatal Cohort). Nearly 20% of participants reported experiencing some kind of SV before young adulthood. Exposure to any kind of SV before young adulthood was not significantly associated with the initial SWL assessment or with SWL at the up to 20-year follow-up. However, exposure to SV participants labelled as rape, incest, and/or serious sexual abuse was significantly associated with lower SWL during young adulthood but not at the midlife follow-up. These results could indicate that exposure to SV may not severely influence SWL over the long term.

Early life predictors of intelligence in young adulthood and middle age.

BackgroundStudies on early predictors of intelligence often focus on single or few predictors and often on childhood intelligence. This study compared the contributions of a broad selection of potential early predictors of intelligence at different adult ages.MethodsInformation on predictors was recorded prospectively in the Copenhagen Perinatal Cohort during pregnancy, at delivery, and at 1- and 3-year examinations for children born between 1959–61. Adult intelligence was assessed at three independent follow-ups using three different tests of intelligence: Børge Priens Prøve, Wechsler Adult Intelligence Scale, and Intelligenz-Struktur-Test 2000R. From a total of 4697 cohort members, three non-overlapping samples were derived.ResultsThe included predictors explained between 22.2–24.3% of the variance in adult IQ, with parental socioeconomic status and sex explaining 16.2–17.0%. Other consistent predictors were head circumference at birth, increase in head circumference head during the first three years, and 3-year milestones. Head circumference was the most important anthropometric measure compared to measures of weight and length.ConclusionBesides social status and sex, the strongest and most consistent early predictors of adult intelligence were physical or behavioural characteristics that to some extent reflect brain–and cognitive development.

A potential link between early language developmental milestones and personality traits in adulthood

Background: Although plausible links between language development and personality have been suggested, longitudinal studies of these associations into adulthood have not been conducted. Aim: To investigate whether children’s age at attaining language milestones is associated with later adult personality. Methods: Mothers’ of 8,400 children from the Copenhagen Perinatal Cohort recorded six language milestones during the child’s first 3 years of life. Information on at least one language milestone was available for two subsamples with adult follow-up information on personality: The Eysenck Personality Questionnaire was available for 691 individuals with a mean age of 27.5 years and The NEO Five-Factor Inventory was available for 902 individuals with a mean age of 50.0 years. Results: Faster attainment of language milestones was associated with lower neuroticism in young adulthood and with higher extraversion and openness to experience in midlife. A total of 1.9% of the variance in neuroticism, 2.5% of the variance in extraversion, and 1.6% of the variance in openness to experience was explained by language milestones. Conclusion: Only a modest part of variance in personality traits was explained by language milestones. However, the study adds to the literature on potential consequences of the timing of language development and suggests a link between language milestones and personality in adulthood.

Predictors of early life milestones: Results from the Copenhagen Perinatal Cohort

BackgroundPre- and postnatal factors have been found to be predictors of age at attaining milestones in infancy; however, the degree to which such factors are predictors of milestones in the subsequent years is less investigated. The aim was to conduct a systematic evaluation of a broad range of possible predictors of milestone attainment during the second and third years to identify factors that explain significant inter-individual variance.MethodsMothers of 4009 children from the Copenhagen Perinatal Cohort (1959–61) were interviewed by a physician about 20 developmental milestones at a three-year examination. Milestones were related to: Language, Walking, Eating, Dressing, Social interaction, and Toilet training. Information on possible predictors was collected during pregnancy and at a 1- and 3-year follow-up.ResultsSeveral pre- and postnatal factors were significantly associated with the timing of milestone attainment; especially parental social status, paternal age, sex, gestational age, birth weight, birth length, weight increase in the first year of life, and motor development during the first year of life. The significant predictors explained 16.2% of the variance in the Overall mean of milestones and 20.3% of the variance in milestones related to Walking. The most influential individual factor for the timing of milestone attainment was previous motor development during the first year of life. Additionally, sex was an important factor as girls were generally faster at attaining milestones. Parental social status was a consistent, but relatively week predictor.ConclusionA notable amount of variance in the timing of milestones during the first three years of life can be explained by perinatal and early postnatal factors. The study provides evidence of developmental continuity as the main predictor of milestones in the second and third years was the speed of development during the first year.

Birth weight and quality of life in midlife: a 50-year follow-up study of 2079 individuals in Denmark.

Low birth weight has been associated with a higher risk of reduced quality of life (QoL) in children, adolescents, and young adults, but the influence seems to diminish over time. However, previous studies have mainly focused on health-related QoL and compared individuals with low birth weight with individuals without low birth weight. The purpose of the present cohort study was to investigate the influence of the entire range of birth weights on three distinct measures of QoL in midlife. The study population consisted of all live-born singletons from the Copenhagen Perinatal Cohort (CPC, 1959-1961) who participated in a 50-year follow-up examination in 2009-2011 (N = 2079). Birth weight was measured by three pediatricians at birth. QoL was measured at the follow-up by the participants' scores on three QoL self-report measures: The Satisfaction With Life Scale, the Vitality Scale of the Medical Outcomes Study 36-Item Short-Form Health Survey, and a single-item QoL measure based on the question: "How is your quality of life at the moment?". General linear regression and binary logistic regression were used to estimate the association between birth weight and QoL in midlife. Small, curvilinear associations of birth weight with life satisfaction, vitality, and the single-item QoL measure were found, suggesting that both low and high birth weights increase the risk of low satisfaction with life, low vitality and low QoL. The study findings suggest that low and high-range birth weight exert a lasting influence on distinct, but complementary aspects of QoL in midlife.

Intelligence in young adulthood and alcohol use disorders in a prospective cohort study of Danish men: the role of psychiatric disorders and parental psychiatric history

ObjectivesThe aims were to estimate the association between intelligence measured in young adulthood and risk of alcohol use disorders (AUD) in men and to investigate the potential modification of this...

Breastfeeding duration in infancy and adult risks of type 2 diabetes in a high-income country.

Observed associations between breastfeeding and reduced risk of type 2 diabetes in adulthood may be confounded. We examined if the duration of breastfeeding in infancy was associated with the risk of type 2 diabetes in adulthood after adjustment for a range of prenatal and postnatal risk factors. We prospectively followed 6,044 individuals from the Copenhagen Perinatal Cohort born 1959-1961. Duration of any breastfeeding (≤0.5, >0.5-1, >1-2, >2-4, >4months) was assessed at the infant's 1-year health examination. We estimated hazard ratios (HRs) with 95% confidence intervals (CIs) for type 2 diabetes (at age ≥30years, 237 persons) by breastfeeding duration without and with adjustment for parental social status and education, maternal pre-pregnancy body mass index (BMI), maternal diabetes and smoking during pregnancy, gestational weight gain, parity, preterm birth, birth weight, sex, and BMI at ages 7 and 41-43years. In the unadjusted analysis, compared with infants breastfed for ≤0.5month, those breastfed for >4months had a 51% reduced risk of type 2 diabetes (HR=0.49; 95% CI [0.32, 0.75]). After the stepwise adjustment for putative early life confounders, this was attenuated to a nonsignificant 31% reduced risk (HR=0.69; 95% CI [0.44, 1.07]). Adjustment for childhood and adulthood BMI minimally changed the results. We found that the inverse association between the duration of breastfeeding and risk of type 2 diabetes in adulthood is considerably weakened and no longer significant after adjustment for prenatal and postnatal factors in the infant and mother.

Maternal infections during pregnancy and offspring midlife inflammation

BackgroundMicrobial exposures early in life have been found to be associated with lower levels of inflammation in adulthood; however, the role of prenatal exposure to infection on offspring inflammatory profiles is unexplored. The aim was to study if maternal infections during pregnancy are associated with inflammation among offspring in later life and to determine if there are sensitive periods of exposure.MethodsThe study was comprised of 1719 participants in the Copenhagen Aging and Midlife Biobank (CAMB) who were also members of the Copenhagen Perinatal Cohort (CPC). When the CPC was established, information on maternal infections during pregnancy was prospectively collected by a trained medical doctor. The inflammatory measures collected in late midlife included, C-reactive protein (CRP), Interleukin-6 (IL-6), TNF-alpha (TNF-α) and Interleukin-10 (IL-10). Multivariable ordinary least squared regression models were implemented to explore associations between maternal infection and inflammatory measures in offspring, controlling for maternal smoking, pre-pregnancy body mass index, age, marital status and parity.ResultsMaternal infection was associated with a 7% lower CRP level (95% CI, − 17,5%) among offspring compared with offspring born to women without an infection and similarly an 8% lower level of IL-6 (95% CI -15,1%), and a 9% lower level of IL-10 (95% CI, − 23,20%). However, differences did not reach significance. The effects of infection during the first trimester did not differ from infections later in the pregnancy.ConclusionsOur results suggested that prenatal exposure to infection may be associated with lower levels of inflammatory markers among adult offspring. Additional prospective studies are needed to further explore this finding.

Infant Socioeconomic Position and Quality of Life in Midlife: A 50-Year Follow-Up Study of 2079 Individuals in Denmark

During the last decades, an important line of public health research has focused on the influence of socioeconomic position (SEP) on the quality of life (QoL) of individuals. While most studies have investigated the association between contemporary SEP and QoL, the present study investigated the association between SEP within the first year of life and QoL in midlife. The study population comprised all live-born singletons from the Copenhagen Perinatal Cohort (CPC; 1959–1961), who participated in a 50-year follow-up examination in 2009–2011 (N = 2079). Infant SEP was measured by a composite SEP indicator in the CPC 1-year examination. QoL was measured in the 50-year examination by the participants’ scores on three different and complementary measures of QoL: The Satisfaction With Life Scale (SWLS) and the Vitality Scale of the MOS 36-Item Short-Form Health Survey (SF-36) as well as by their answers to the question: “How is your quality of life at the moment?”. Linear regression and ordinal logistic regression were used to estimate the association between infant SEP and QoL in midlife. The results showed that infant SEP was positively associated with all three measures of QoL in midlife, albeit only the associations with the SWLS and the Vitality Scale were statistically significant. In conclusion, infant SEP appears to exert a lasting influence on QoL in midlife.

Language development and intelligence in midlife.

Individual differences in early language skills have been found to be associated with other cognitive outcomes in childhood and adolescence. However, research is limited on whether these associations persist into adulthood. In this study, we examined potential associations of the timing of early language milestones with cognitive ability in a prospective cohort study of 938 singletons from the Copenhagen Perinatal Cohort (CPC), who participated in a 50-year follow-up examination in 2009-2011. Later attainment of a number of milestones was associated with lower midlife IQ with the strongest associations found for 'Naming objects/animals in pictures', 'Forming a sentence', and 'Sharing experiences'. Milestones related to language explained 6.7% of the variance in midlife IQ, while milestones related to social interaction explained 3.1%. Thestudy provides evidence that individual differences in language development during the first years of life are associated with intelligence in midlife. Statement of contribution What is already known on this subject? Differences in early language skills are associated with other cognitive skills in childhood and adolescence. No study has examined this association from childhood through midlife in a large community-based sample. What does this study add? Early language development is associated with intelligence in midlife. A total of 6.7% of the variance in midlife IQ is explained by milestones related to language. Adjustment for potentially confounding factors did not change the associations.

Parental socioeconomic position and midlife allostatic load: a study of potential mediators

BackgroundThe mechanisms underlying the association of parental socioeconomic position with later life allostatic load remain unclear. The present study aims to examine potential pathways underlying this association: personality, social relations, intelligence and education.MethodsThe study comprised 361 members of the Copenhagen Perinatal Cohort who participated in two subsequent follow-ups: the Prenatal Development Project (mean age 27 years) and the Copenhagen Aging and Midlife Biobank study (mean age 50 years). Allostatic load was based on 14 biomarkers representing the inflammatory, metabolic and cardiovascular system measured at midlife. Information on potential mediators was collected in young adulthood, and their role in the association of parental socioeconomic position with midlife allostatic load were examined in linear regression path analyses.ResultsParental socioeconomic position at one year was inversely associated with midlife allostatic load (β = − 0.238, p < .001). No mediation effects were found for personality or social relations. In a model including intelligence and education, a significant indirect effect was found for education (β = − 0.151, p < .001). A significant direct effect remained (β = − 0.111, p = .040).ConclusionsParental socioeconomic position was inversely associated with allostatic load in midlife. Results suggest that part of this association was mediated by education. A better understanding of the non-cognitive pathways related to education is an important prerequisite for the development of effective intervention strategies.

Early life predictors of midlife allostatic load: A prospective cohort study.

BackgroundAllostatic load has been suggested as a pathway through which experiences become biologically embedded to influence health. Research on childhood predictors of allostatic load has focused on socioeconomic and psychosocial exposures, while few studies include prospective measures of biomedical exposures. Further, findings on sex differences in the association of childhood predictors with various health outcomes related to allostatic load are ambiguous.AimsTo examine the influence of early life biomedical and social factors in the first year of life on midlife allostatic load, assessing potential sex differences.MethodsThis prospective cohort study includes early life information collected at birth and a one year examination for 1,648 members of the Copenhagen Perinatal Cohort who also participated in the Copenhagen Aging and Midlife Biobank study (aged 49–52 years, 56% women). Allostatic load based on 14 biomarkers was selected as a measure of midlife health status. Early life factors were categorized as predominantly biomedical or social, and their associations with midlife allostatic load were examined in domain-specific and combined sex-stratified multiple regression models.ResultsThe biomedical factors model explained 6.6% of the variance in midlife allostatic load in men and 6.7% in women, while the social model explained 4.1% of the variance in men and 7.3% in women. For both sexes, parental socioeconomic position at one year and maternal BMI significantly predicted midlife allostatic load in a model containing all early life factors. For women, additional significant predictors were complications at birth, birth weight and not living with parents at one year.ConclusionThe results confirm an association of lower childhood socioeconomic position with higher adult allostatic load while demonstrating the importance of other prenatal and early life exposures and highlighting potential sex differences.

Developmental milestones during the first three years as precursors of adult intelligence.

Few studies have investigated associations of milestone development in early childhood with intelligence in adulthood in typically developing children. The current study is an extension of 2 previous studies on smaller samples and investigated associations of age at attainment of 32 developmental milestones attained between 0 and 3 years of age with adult intelligence and explored whether the effects of early infant milestones are mediated through later development during subsequent years. Mothers of 8,400 children from the Copenhagen Perinatal Cohort recorded 32 developmental milestones during the child's first 3 years of life. Information on at least 1 developmental milestone was available for 2 subsamples with adult follow-up information on intelligence: Børge Priens Prøve (BPP) was available for 2,567 men while Wechsler Adult Intelligence Scale (WAIS) was available for 1,000 men and women. The study found that early attainment of milestones, primarily those related to standing/walking and language development, was associated with higher intelligence in adulthood. The adjusted BPP means were 103.7, 101.7, and 99.5, respectively, for being able to name objects/animals in pictures at less than 18 months, 18-24 months, and later than 24 months. Mediation analyses showed direct associations of the 1-year milestones related to standing/walking with intelligence with the direct association accounting for 74.6% and 64.4% of the total association in each subsample. Thus, milestones related to standing and walking primarily show a direct association with adult intelligence and are to a smaller extent mediated by milestones reflecting development during the subsequent years, in particular language development. (PsycINFO Database Record

Prenatal and early postnatal stress and later life inflammation

BackgroundEvidence suggests that maternal psychological and social stress during the prenatal period and in childhood represent an important condition that may adversely impact the anatomy and physiology of the developing child with implications for a number of health-related conditions and disorders. In a large prospective study, we aim to address if social stressors in the prenatal and early postnatal periods, as individual exposures as well as their accumulation, are associated with a range of inflammatory markers in late middle-aged offspring. MethodsThe study sample includes Danish men and women born between 1959 and 1961 (n = 1206) who were members of the Copenhagen Perinatal Cohort and participated in the Copenhagen Aging and Midlife Biobank in 2009–2011 (age 49–52). Information on social stressors was collected through an interview with the mothers at the first antenatal visit and postnatal stressor data was collected at year one follow-up. A series of ordinary least square regression models were performed with the stress measures as the exposures and C-reactive protein (CRP), Interleukin-6 (IL-6), Interleukin-10 (IL-10), and Tumor necrosis factor α (TNF-α) separately as the outcomes. ResultsThe individual prenatal maternal stressors (being unmarried and having an unwanted pregnancy) and the prenatal index were associated with higher levels of CRP and IL-6 among offspring but not with IL-10 or TNF-α. Low social status, but not living away from parents or having an unmarried mother in the first year of life, was associated with higher levels of CRP and IL-6. The accumulation of social stressors in the early postnatal period was associated with higher levels of CRP and IL-6 but not IL-10 and TNF-α. The accumulation of stressors in the prenatal and postnatal periods combined was associated with higher levels of CRP and IL-6, but not with IL-10 or TNF-α. ConclusionsThe findings suggest that exposure to the accumulation of prenatal and early life stressors, is associated with higher levels of CRP and IL-6 in later life. This may indicate that the effects of early stressors on later inflammation operate through pathways with clear links to cardiovascular disease.

Birth Weight and Intelligence in Young Adulthood and Midlife

We examined the associations between birth weight and intelligence at 3 different adult ages. The Copenhagen Perinatal Cohort is comprised of children born in Copenhagen from 1959 to 1961. Information on birth weight and ≥1 tests of intelligence was available for 4696 members of the cohort. Intelligence was assessed at a mean age of 19 years with the Børge Priens Prøve test, at age 28 years with the Wechsler Adult Intelligence Scale, and at age 50 years with the Intelligenz-Struktur-Test 2000 R. Birth weight was significantly associated with intelligence at all 3 follow-up assessments, with intelligence scores increasing across 4 birth weight categories and declining for the highest birth weight category. The adjusted differences between those in the <2.5kg birth weight group and those in the 3.5 to 4.00kg group were >5 IQ points at all 3 follow-up assessments, corresponding to one-third of a SD. The association was stable from young adulthood into midlife,and not weaker at age 50 years. Adjustment for potential confounding factors, including infant socioeconomic status and gestational age, did not dilute the associations, and associations with intelligence were evident across the normal birth weight range and so were not accounted for by low birth weight only. The association between birth weight and intelligence is stable from young adulthood into midlife. These long-term cognitive consequences may imply that even small shifts in the distribution of birth size, in normal-sized infants as well, may have a large impact at the population level.

Associations of Early Developmental Milestones With Adult Intelligence.

The study investigated whether age at attainment of 20 developmental milestones within the areas of language, walking, eating, dressing, social interaction, and toilet training was associated with adult intelligence. Mothers of 821 children of the Copenhagen Perinatal Cohort recorded 20 developmental milestones at a 3-year examination, and all children were administered the Wechsler Adult Intelligence Scale when they were 20-34years old. Later attainment of a number of milestones was associated with lower adult IQ with the strongest associations found for those related to language and social interaction. The adjusted full-scale IQ means were 107.0, 101.8, and 100.6 for being able to form a sentence at less than 24months, at 24months, and later than 24months.

Childhood pegboard task predicts adult-onset psychosis-spectrum disorder among a genetic high-risk sample

Motor abnormalities have been established as a core aspect of psychosis-spectrum disorders, with numerous studies identifying deficits prior to clinical symptom presentation. Additional research is needed to pinpoint standardized motor assessments associated with psychosis-spectrum disorders prior to illness onset to enhance prediction and understanding of etiology. With a long history of findings among people with diagnosable psychosis-spectrum disorders, but little research conducted during the premorbid phase, pegboard tasks are a viable and understudied measure of premorbid for psychosis motor functioning. In the current study, examining data from the Copenhagen Perinatal Cohort, the Simultaneous Pegs Test was performed with children (n=244, aged 10–13) at genetic high risk for psychosis (n=94) and controls (n=150). Findings suggest that children who eventually developed a psychosis-spectrum disorder (n=33) were less likely to successfully complete the task within time limit relative to controls (χ2(2, N=244)=6.94, p=0.03, ϕ=0.17). Additionally, children who eventually developed a psychosis-spectrum disorder took significantly longer to complete the task relative to controls (χ2(2, N=244)=7.06, p=0.03, ϕ=0.17). As pegboard performance is thought to tap both diffuse and specific brain networks, findings suggest that pegboard tests may be useful premorbid measures of motor functioning among those on a trajectory towards a psychosis-spectrum disorder.

Infant developmental milestones and adult intelligence: A 34-year follow-up

BackgroundA number of studies suggest a positive association between faster infant motor development and intellectual function in childhood and adolescence. However, studies investigating the relationship between infant motor development and intelligence in adulthood are lacking. AimsTo investigate whether age at achievement of 12 motor developmental milestones was associated with adult intelligence and to evaluate the influence of sex, parental social status, parity, mother's cigarette consumption in the last trimester, gestational age, birthweight, and birth length on this association. MethodsMothers of 9125 children of the Copenhagen Perinatal Cohort recorded 12 developmental milestones during the child's first year of life. A subsample of the cohort comprising 1155 individuals participated in a follow-up when they were aged 20–34 years and were administered the Wechsler Adult Intelligence Scale (WAIS). Associations between motor developmental milestones and IQ were analysed by multiple linear regression adjusting for potential confounding factors. ResultsLater acquisition of infant developmental milestones was associated with lower subsequent IQ, and the majority of significant associations were found for Performance IQ. Correlations were generally small (r<0.10), but significant interactions were found between parental social status and age of attaining developmental milestones, with associations being significantly stronger in the offspring of lower social status parents. The effects remained significant after adjusting for possible confounding factors. ConclusionThis is the first study to find significant interactions with parental social status, thereby suggesting that associations between early motor development and intelligence are stronger in infants of low social status parents.