Later-born Children Research Articles

Changing Effects of Birth Order on Education Over Time: Evidence From Cambodia

Despite the vast literature on birth-order effects in developing countries, there has been little research on their long-term changes. This study uses three population censuses of Cambodia to investigate how birth order affects educational attainments and how its effects changed over time. We estimate regressions with family fixed effects and cohort fixed effects, and present four findings. First, later-born children are less likely to enroll in primary school and gain literacy skills, and end up with fewer years of schooling than first-born children. Second, controlling for gender differences in education, we find smaller birth-order effects for girls. Third, there is a strong relationship between the gender composition of siblings and birth-order effects. Finally, not only have these birth-order effects attenuated over time but the relationship between the gender composition of siblings and birth-order effects has also changed. In the 1998 census, if the firstborn child is male, his sisters’ educational outcomes are influenced more by their birth order compared with his brothers. In contrast, in the 2019 census, if the firstborn child is male, his sisters' educational outcomes are less susceptible to birth-order effects compared with his brothers. Our findings indicate gendered economic disparities within households and their long-term changes.

Effects of Siblings on Cognitive and Sociobehavioral Development: Ongoing Debates and New Theoretical Insights

Despite social scientists’ long-standing interest in the influences of siblings, previous research has not settled the debates on how relevant sibship size is to child development and whether growing up with more siblings could be beneficial. Using 30 years of longitudinal data and fixed-effects models, this study offers the most comprehensive evidence on how sibship size is tied to cognitive and sociobehavioral development. We also advance the literature by systematically comparing the consequences of gaining a sibling for children with varying ordinal positions. Contrary to prior studies using selective data from limited observation spans, we find that children experience net decreases in cognitive test scores as their family size grows. At the same time, our analysis shows that sibling additions are only important to first- and second-born children’s—not later-born children’s—cognitive development. Even for the first- and second-born, the marginal effect of adding a sibling lessens with each addition. Our results thus demonstrate the time-dependent nature of family resource-dilution processes. For sociobehavioral development, the evidence indicates that having an older sibling is beneficial, but gaining a younger sibling increases behavioral problems for some (e.g., first-born children). Because more children from large families have older siblings, children from larger families exhibit less problematic behavior, on average. By uncovering the complex relationship between siblings and noncognitive development, this study also generally contributes to the sociology of family and inequality.

Maternal Factors in Relation to Early Childhood Caries Among Preschool Children in Myanmar.

A cross-sectional study was conducted among 3- to 5-year-old preschool children in Yangon City in 2016-2017 to investigate the association between their caries experience and maternal factors. Children's oral examination and questionnaire surveys to their mothers were conducted. Children's caries prevalence was 82.9% with mean dmft of 6.4 ± 4.9. Mean oral health knowledge score of their mothers was 8.5 ± 2.8. Mother's educational level and oral health knowledge were significant predictors of childhood caries. Third- or later-born children were twice more likely to have dental caries than first-born children (P = .022). Mother's regular dental check-up resulted in significantly lower odds ratios for caries development in her child. Children's caries experience was high and mother's oral health knowledge regarding the importance of primary teeth was unsatisfactory. It is, therefore, necessary to improve awareness and attentiveness for prevention of childhood caries among parents and guardians in Myanmar.

FAMILY FLYNN EFFECTS AND LINKS TO MIDDLE-AGE HEALTH OUTCOMES

Abstract The Flynn effect (Flynn, 1984; 1987) refers to increases in cognitive performance, for later-born cohorts. It has been documented globally, occurring for more than a century. In a meta-analysis, Pietschnig and Voracek (2015) noted that the effect may be even stronger in adults than in children, though little research has addressed this topic (or its implications) for aging adults. Similarly, overall life-time health has improved, and incidences of cognitive impairment have decreased during the last two decades (Clouston et al., 2021). Using multilevel growth curve models, we found family Flynn effects in the National Longitudinal Survey of Youth; children in families with later-born mothers, and later-born first children, had higher PIAT math scores, and steeper developmental slopes. Although the link from childhood and adolescent cognitive function to later life outcomes has been well studied, research that takes advantage of the Flynn effect to facilitate interpreting that link is lacking. Clouston et al. (2021) emphasized the value of the Flynn effect in investigating links between childhood cognitive functioning and later adult Alzheimer’s disease and related dementia (ADRD) risks. We linked our family level results to middle-age maternal health outcomes (factors that are related to ADRD risks). Canonical correlation analyses showed that mothers (at ages 40+ and 50+) from families with higher score levels and slopes tended to have better mental and physical health. Our results, showing a Flynn effect in child and adolescence scores, at the family level, with links to adult health, persisted after controlling for a known selection bias.

Development of a Model to Determine the Purchase Intention of Counterfeit Luxury Products Among Consumers in Bengaluru

The luxury fashion industry is a profitable one and is expected to grow at a rate of 10.99% annually. With the luxury industry growing, the counterfeiting industry is also seeing rapid growth. India’s trade has increased by 24% in the last two years. The research attempted to investigate the factors affecting consumers’ attitudes toward the purchase intention of counterfeit luxury products in South-East Bengaluru. The primary data were collected through an online survey questionnaire. Using the cluster sampling technique, a total of 309 responses were received, of which seven were rejected due to incomplete responses. Therefore, a sample size of 302 was seen fit to use for statistical purposes. Based on the identified factors, initially, a conceptual framework was developed, and the model was tested using JMP Pro 16 software. The research findings revealed that low -income group consumers tended to buy counterfeits more often. The ordinal position of birth played a significant role where first-born children showcased an unfavorable attitude. However, later-born children showcased a more positive attitude. Young adults showed an approved attitude toward counterfeits. The social class of students and employees showed an encouraging attitude toward counterfeits. Several personal and social factors were found to have a strong influence on the attitude toward counterfeits. From the findings, suggestions and recommendations have been drawn to assist genuine brands to frame better tactics to curb counterfeiting.

A Closer Look at the Birth Order Effect on Early Cognitive and School Readiness Development in Diverse Contexts.

Using the Early Childhood Longitudinal Study–Birth Cohort data, we examined the effect of birth order (firstborn vs. later-born) on children's cognitive skills at 24 months and school readiness (i.e., math and literacy) skills at age 4 years. Previous research in the U.S. using predominantly English-speaking, low-risk samples suggests that firstborns tend to show better early cognitive and school readiness skills than later-born children. However, results of the current study showed that although there was a firstborn advantage in low-risk or English-speaking families, in high-risk or language minority families, later-born children showed equivalent or even better skills than firstborn children. Our moderated mediation models revealed that children's engagement in home learning activities mediated the relation between birth order and developmental outcomes, and families' cumulative risks and language minority status moderated the mediation pathways. These findings underscore the complex associations between birth order and early development in diverse ecological contexts.

The effect of birth order on entrepreneurship: evidence from China

ABSTRACT Recent studies on the determinants of entrepreneurship have shown that later-born children are more likely to become entrepreneurs. However, research has not addressed the questions of how birth order influences entrepreneurship. Based on the Chinese context, we propose two competing hypotheses to explore the potential mechanisms (risk taking vs. educational attainment) that explain the effect of birth order on entrepreneurship. We further argue that the effect of birth order on entrepreneurship is moderated by birth spacing and family financial status. Using data from the 2008 and 2013 Chinese Household Income Project, we find that in the case of China the impacts of birth order on two types of entrepreneurship (i.e. solo entrepreneurship and employer entrepreneurship) are overall positive, which implies that the risk-taking mechanism is more dominant than the educational attainment mechanism. Birth order shows a more pronounced impact for solo entrepreneurship but a weak impact for employer entrepreneurship. We also find that the positive birth-order effect on entrepreneurship is more evident in families with low financial status. Overall, this paper casts light on how birth order shapes the propensity for entrepreneurship.

Endogenous inclusion in the Demographic and Health Survey anthropometric sample: Implications for studying height within households

Development economists study both anthropometry and intra-household allocation. In these literatures, the Demographic and Household Surveys (DHS) are essential. The DHS censors its anthropometric sample by age: only children under five are measured. We document several econometric consequences, especially for estimating birth-order effects. Child birth order and mothers’ fertility are highly correlated in the age-censored anthropometric subsample. Moreover, family structures and age patterns that permit within-family comparisons of siblings’ anthropometry are unrepresentative. So strategies that could separate birth order and fertility in other data cannot here. We show that stratification by mother’s fertility is important. We illustrate this by comparing India and sub-Saharan Africa (SSA). Children in India born to higher-fertility mothers are shorter, on average, than children of lower-fertility mothers. Yet, later-born children in India are taller, adjusted for age, than earlier-born children of the same sibsize. In SSA, neither of these associations is large.

Birth order and prosociality in the early adolescent brain

Birth order is a crucial environmental factor for child development. For example, later-born children are relatively unlikely to feel secure due to sibling competition or diluted parental resources. The positive effect of being earlier-born on cognitive intelligence is well-established. However, whether birth order is linked to social behavior remains controversial, and the neural correlates of birth order effects in adolescence when social cognition develops remain unknown. Here, we explored the birth order effect on prosociality using a large-scale population-based adolescent cohort. Next, since the amygdala is a key region for sociality and environmental stress, we examined amygdala substrates of the association between birth order and prosociality using a subset neuroimaging cohort. We found enhanced prosociality in later-born adolescents (N = 3160), and observed the mediating role of larger amygdala volume (N = 208) and amygdala-prefrontal functional connectivity with sex-selective effects (N = 183). We found that birth order, a non-genetic environmental factor, affects adolescent social development via different neural substrates. Our findings may indicate the later-born people’s adaptive survival strategy in stressful environments.

Small-quantity lipid-based nutrient supplements for children age 6–24 months: a systematic review and individual participant data meta-analysis of effects on developmental outcomes and effect modifiers

ABSTRACTBackgroundSmall-quantity (SQ) lipid-based nutrient supplements (LNSs) provide many nutrients needed for brain development.ObjectivesWe aimed to generate pooled estimates of the effect of SQ-LNSs on developmental outcomes (language, social-emotional, motor, and executive function), and to identify study-level and individual-level modifiers of these effects.MethodsWe conducted a 2-stage meta-analysis of individual participant data from 14 intervention against control group comparisons in 13 randomized trials of SQ-LNSs provided to children age 6–24 mo (total n = 30,024).ResultsIn 11–13 intervention against control group comparisons (n = 23,588–24,561), SQ-LNSs increased mean language (mean difference: 0.07 SD; 95% CI: 0.04, 0.10 SD), social-emotional (0.08; 0.05, 0.11 SD), and motor scores (0.08; 95% CI: 0.05, 0.11 SD) and reduced the prevalence of children in the lowest decile of these scores by 16% (prevalence ratio: 0.84; 95% CI: 0.76, 0.92), 19% (0.81; 95% CI: 0.74, 0.89), and 16% (0.84; 95% CI: 0.76, 0.92), respectively. SQ-LNSs also increased the prevalence of children walking without support at 12 mo by 9% (1.09; 95% CI: 1.05, 1.14). Effects of SQ-LNSs on language, social-emotional, and motor outcomes were larger among study populations with a higher stunting burden (≥35%) (mean difference: 0.11–0.13 SD; 8–9 comparisons). At the individual level, greater effects of SQ-LNSs were found on language among children who were acutely malnourished (mean difference: 0.31) at baseline; on language (0.12), motor (0.11), and executive function (0.06) among children in households with lower socioeconomic status; and on motor development among later-born children (0.11), children of older mothers (0.10), and children of mothers with lower education (0.11).ConclusionsChild SQ-LNSs can be expected to result in modest developmental gains, which would be analogous to 1–1.5 IQ points on an IQ test, particularly in populations with a high child stunting burden. Certain groups of children who experience higher-risk environments have greater potential to benefit from SQ-LNSs in developmental outcomes.This trial was registered at www.crd.york.ac.uk/PROSPERO as CRD42020159971.

Characteristics that modify the effect of small-quantity lipid-based nutrient supplementation on child growth: an individual participant data meta-analysis of randomized controlled trials

ABSTRACTBackgroundMeta-analyses show that small-quantity lipid-based nutrient supplements (SQ-LNSs) reduce child stunting and wasting. Identification of subgroups who benefit most from SQ-LNSs may facilitate program design.ObjectivesWe aimed to identify study-level and individual-level modifiers of the effect of SQ-LNSs on child growth outcomes.MethodsWe conducted a 2-stage meta-analysis of individual participant data from 14 randomized controlled trials of SQ-LNSs provided to children 6–24 mo of age (n = 37,066). We generated study-specific and subgroup estimates of SQ-LNS compared with control and pooled the estimates using fixed-effects models. We used random-effects meta-regression to examine study-level effect modifiers. In sensitivity analyses, we examined whether results differed depending on study arm inclusion criteria and types of comparisons.ResultsSQ-LNS provision decreased stunting (length-for-age z score < −2) by 12% (relative reduction), wasting [weight-for-length (WLZ) z score < −2] by 14%, low midupper arm circumference (MUAC) (<125 mm or MUAC-for-age z score < −2) by 18%, acute malnutrition (WLZ < −2 or MUAC < 125 mm) by 14%, underweight (weight-for-age z score < −2) by 13%, and small head size (head circumference-for-age z score < −2) by 9%. Effects of SQ-LNSs generally did not differ by study-level characteristics including region, stunting burden, malaria prevalence, sanitation, water quality, duration of supplementation, frequency of contact, or average compliance with SQ-LNS. Effects of SQ-LNSs on stunting, wasting, low MUAC, and small head size were greater among girls than among boys; effects on stunting, underweight, and low MUAC were greater among later-born (than among firstborn) children; and effects on wasting and acute malnutrition were greater among children in households with improved (as opposed to unimproved) sanitation.ConclusionsThe positive impact of SQ-LNSs on growth is apparent across a variety of study-level contexts. Policy-makers and program planners should consider including SQ-LNSs in packages of interventions to prevent both stunting and wasting.This trial was registered at www.crd.york.ac.uk/PROSPERO as CRD42019146592.

Characteristics that modify the effect of small-quantity lipid-based nutrient supplementation on child anemia and micronutrient status: an individual participant data meta-analysis of randomized controlled trials

ABSTRACTBackgroundSmall-quantity lipid-based nutrient supplements (SQ-LNSs) have been shown to reduce the prevalence of child anemia and iron deficiency, but effects on other micronutrients are less well known. Identifying subgroups who benefit most from SQ-LNSs could support improved program design.ObjectivesWe aimed to identify study-level and individual-level modifiers of the effect of SQ-LNSs on child hemoglobin (Hb), anemia, and inflammation-adjusted micronutrient status outcomes.MethodsWe conducted a 2-stage meta-analysis of individual participant data from 13 randomized controlled trials of SQ-LNSs provided to children 6–24 mo of age (n = 15,946). We generated study-specific and subgroup estimates of SQ-LNSs compared with control, and pooled the estimates using fixed-effects models. We used random-effects meta-regression to examine potential study-level effect modifiers.ResultsSQ-LNS provision decreased the prevalence of anemia (Hb < 110 g/L) by 16% (relative reduction), iron deficiency (plasma ferritin < 12 µg/L) by 56%, and iron deficiency anemia (IDA; Hb < 110 g/L and plasma ferritin <12 µg/L) by 64%. We observed positive effects of SQ-LNSs on hematological and iron status outcomes within all subgroups of the study- and individual-level effect modifiers, but effects were larger in certain subgroups. For example, effects of SQ-LNSs on anemia and iron status were greater in trials that provided SQ-LNSs for >12 mo and provided 9 (as opposed to <9) mg Fe/d, and among later-born (than among first-born) children. There was no effect of SQ-LNSs on plasma zinc or retinol, but there was a 7% increase in plasma retinol-binding protein (RBP) and a 56% reduction in vitamin A deficiency (RBP < 0.70 µmol/L), with little evidence of effect modification by individual-level characteristics.ConclusionsSQ-LNSs can substantially reduce the prevalence of anemia, iron deficiency, and IDA among children across a range of individual, population, and study design characteristics. Policy-makers and program planners should consider SQ-LNSs within intervention packages to prevent anemia and iron deficiency.This trial was registered at www.crd.york.ac.uk/PROSPERO as CRD42020156663.

Association Between the COVID-19 Pandemic and Infant Neurodevelopment: A Comparison Before and During COVID-19.

Aim: To investigate the association between the experience of the coronavirus disease 2019 (COVID-19) pandemic and neurodevelopment of 6-month-old and 1-year-old children and explore the differences in the association by birth order.Methods: This comparison study was embedded in the Born in Guangzhou Cohort Study in China. The exposed group included 546 6-month-old and 285 1-year-old children who attended neurodevelopment assessments between March 1 and May 15, 2020, and the non-exposed group included 3,009 6-month-old and 2,214 1-year-old children during the same months from 2015 to 2019. Neurodevelopment at age 6 months and 1 year was assessed by trained clinical staff using the Ages and Stages Questionnaires, third edition (ASQ-3) and the Gesell Developmental Schedules (GDS).Results: The experience of the pandemic in 2020 was associated with a higher risk of delay in the fine motor (adjusted OR: 2.50, 95% CI: 1.25, 4.99; estimated by logistic regression) and communication (adjusted RR [aRR]: 1.13, 95% CI: 1.02, 1.25; estimated by log-binomial regression) domains at age 1 year. The association between the experience of the pandemic and communication delay at age 1 year only existed in first-born children (aRR: 1.15, 95% CI: 1.03, 1.30) but not in later-born children (aRR: 1.02, 95% CI: 0.84, 1.25). No associations were observed in any domain among 6-month-olds.Conclusion: Experiencing the COVID-19 pandemic and related public health strategies might be associated with a higher risk of delay in the development of fine motor and communication in 1-year-old children; the association observed in the communication domain only existed in first-born children.

Sharing the pie: An analysis of undernutrition and individual consumption in Bangladesh

Anti-poverty policies often aim to reach poor individuals by targeting poor households. However, intra-household inequality may mean some poor individuals reside in non-poor households. Using Bangladeshi data, we first show that undernourished individuals are spread across the household per-capita expenditure distribution. We then quantify the extent of total consumption inequality within families. We apply a novel approach to identify individual-level consumption within a collective household model and use the structural estimates to compute poverty rates separately for women, men, boys, girls, and the elderly. We find that women (especially older women) and children (later-born children in particular) face significant probabilities of living in poverty even in households with per-capita expenditure above the poverty line. This poverty misclassification is severe, as one third of poor individuals in our sample live in non-poor families.

Parental Investment Is Biased toward Children Named for Their Fathers.

Namesaking (naming a child after a parent or other relative) can be viewed as a mechanism to increase perceived parent-child similarity and, consequently, parental investment. Male and, to a lesser extent, firstborn children are more frequently namesakes than female and later-born children, respectively. However, a direct link between namesaking and parental investment has not been examined. In the present study, 632 participants (98 men and 534 women) from Central Europe indicated their first name, sex, birth order, number of siblings, sexual orientation, socioeconomic status, paternal and maternal first names, as well as relationship quality with, and time and financial investment they received from, both parents during childhood. Mixed-effects models revealed associations between namesaking and parental investment. However, the effect of namesaking often appeared significant only in interaction with specific predictors, such as sex and primogeniture. It suggests instead that namesaking has an additive effect-it enhances the effect of biological factors on parental investment. In general, we found evidence for the bias in parental investment linked to name similarity among both parents, and support for the hypothesis that namesaking serves as a mechanism to increase paternity confidence and, thus, paternal investment. The effect of namesaking influences only certain types of parental investment-namely, those at the level of relationship quality. In addition, nonheterosexual orientation was the strongest negative predictor of paternal investment. Our study extends the research on parental investment by showing that cultural mechanisms, such as namesaking, can also exert some influence on parental rearing behavior.

Parent engagement with the New South Wales child health home-based record: A cross-sectional survey and a retrospective chart review.

In 2018, World Health Organization endorsed universal use of home-based records to improve care for mothers, pregnant women, newborns and children. New South Wales (NSW), Australia has had universal use of a child health home-based record since 1988, with the first major update in 2013. Since the update, limitedbelief is sufficient for constituting evidence has been collected about factors influencing parent use of the record. This study aims to examine parent engagement with the record and whether it is influenced by child's first-born status and Parent Evaluation of Development Status (PEDS) outcome. A survey of 202 parent-child pairs and a review of 20 record books were conducted in NSW, Australia. Odds ratio (OR) at 95% confidence interval (CI) and .05 level of significance, bivariate and multivariate logistic regressions were conducted to examine the influence of first-born status, child's PEDS outcome and other parent-child characteristics on parent engagement with the record. Parents engaged with the home-based record by taking it for routine checks (80.7%), writing (74.8%) or reading (75.8%) information. Parents of first-born children were more than three times likely to take the record book for routine checks compared with parent of later-born children (adjusted OR [AOR] = 3.70, 95% CI 1.12-12.20). Similarly, parents of children with low PEDS level of risk were significantly less likely to read information in the record book compared with parents of children with high risk (AOR = 0.28, 95% CI 0.08-0.98). Parent record input through direct observation significantly decreased from 85% at 1-month to 60% at 12-month postnatal. Data about at least one vaccination event were recorded in all the record books examined, while 85% had all recommended vaccinations recorded. Parents valued/engaged regularly with a child health home-based record, and the record may be a useful tool for early detection and monitoring of child developmental concerns. Therefore, continued promotion of the use of child health home-based record appears to play an important role in improving child health and development.

Birth Order and Divergent Thinking: A Meta-Analysis

ABSTRACT This meta-analysis examined the association of birth order and divergent thinking (DT). The main purpose was to examine how ordinal position (only, first, middle, or last-born) is related to creativity. The results from 27 studies (k= 222; N = 4,690) were analyzed using a multilevel approach. Because some previous studies compared first- vs. later-born children whereas others compared different ordinal positions, three datasets were analyzed: (a) first-born vs. later-born children, (b) only children vs. children with siblings, and (c) middle-born vs. later-born children. Results showed that first-borns had higher DT scores than later-born children (g = −0.165, 95% CI [−0.012, −0.318], p = .046). Moderator analysis showed a significant interaction between gender and task modality with later-born males scoring lower on figural (but not verbal) DT than first-born males. No significant difference was found between only children and first-born children with siblings. In addition, the results showed a non-significant difference between middle- and later-born children; however, moderator analysis indicated that gender and age significantly explained some of the variability in the mean effect. Limitations and practical implications of the findings are discussed.

Effects of short birth spacing on birth-order differences in child stunting: Evidence from India

Do firstborn children have a height advantage? Empirical findings have found mostly that, yes, second or higher-order children often lag behind firstborns in height outcomes, especially in developing countries. However, empirical investigations of birth-order effects on child height overlook the potential impact that birth spacing can have. We provide an explanation for the negative birth-order effect on stunting outcomes for young Indian children and show it is driven by short preceding-birth spacing. We find that firstborn children are taller than children of higher birth order: The height-for-age gap for third (or higher)-order children is twice the gap for children second in birth order. However, this pattern is observed when spacing between later-born children and their immediate elder siblings is fewer than 3 y. Interestingly, the firstborn height advantage disappears when later-born children are born at least 3 y after their elder siblings. Thus, our findings indicate that spacing length between children explains differences in height, over birth order. Although India's family planning policy has resulted in a substantial reduction in total fertility, its achievement in spacing subsequent births has been less impressive. In showing that spacing can alleviate or aggravate birth-order effects on attained height, our study fills an evidence gap: Reducing fertility alone may not be sufficient in overcoming negative birth-order effects. To reduce the detrimental effects of birth order on child stunting, policy responses-and therefore research priorities-require a stronger focus on increasing the time period between births.

Birth order, parental health investment, and health in childhood

We analyze the relationship between birth order, parental health investment and children's health using administrative data from Austria. We show that later-born children have better health endowments at birth. They are less likely born preterm or with a low birth weight, and less likely hospitalized for perinatal conditions. We also find significant birth order differences in parental health investment in early childhood. Later-born children are less likely to participate in preventive medical screenings and their vaccine uptake rates are lower. Our analysis indicates that these birth order differences in parental health investments are not driven by children's health endowments. Thus, we do not find evidence for compensatory behavior of parents. We discuss alternative explanations, such as the role of resource constraints. Furthermore, we show, that the initial health inequalities extend into middle childhood. Later-born children show a better health status in school health checks, they consume less medication and are less often hospitalized.

Investigating associations between birth order and autism diagnostic phenotypes.

Birth order effects have been linked to variability in intelligence, educational attainment and sexual orientation. First- and later-born children have been linked to an increased likelihood of an Autism Spectrum Disorder (ASD) diagnosis, with a smaller body of evidence implicating decreases in cognitive functioning with increased birth order. The present study investigated the potential association between birth order and ASD diagnostic phenotypes in a large and representative population sample. Data were obtained from an ongoing prospective diagnostic registry, collected between 1999 and 2017, including children (1-18years of age, n=5,404) diagnosed with ASD in the state of Western Australia. Children with ASD were ranked relative to sibling's birth to establish birth order within families at time of ASD diagnosis. Information reported to the registry by health professionals at the time of diagnostic evaluation included demographic and family characteristics, functional abilities and intellectual capacity. Adaptive functioning and intelligence scores decreased with increasing birth order, with later-born children more likely to have an intellectual disability. Compared to first-born children with siblings, first-born children without siblings at the time of diagnosis also exhibited decreased cognitive functioning. These findings demonstrate for the first time an association between increasing birth order and variability in ASD clinical phenotypes at diagnosis, with potential evidence of reproductive curtailment in children without siblings. Taken together, these findings have significant implications for advancing understanding about the potential mechanisms that contribute to heterogeneity in ASD clinical presentations as a function of birth order and family size.