Later-born Siblings Research Articles

Early gesture development as a predictor of autism spectrum disorder in elevated-likelihood infants of ASD

BackgroundGesture difficulties have been reported in later-born siblings of children with autism spectrum disorder (ASD). Careful observation of gesture development during the first two years of children at elevated likelihood (EL) of developing ASD may identify behavioral indicators that facilitate early diagnosis.MethodsThis study enrolled 47 EL infants and 27 low-likelihood (LL) infants to explore gesture developmental trajectories and the predictive value of gesture to expedite the early detection of core characteristics of ASD. Gesture frequency, communication function, and integration ability were observed and coded from a semi-structured assessment administered longitudinally across 9–19 months of age. We conducted the Autism Diagnostic Observation Schedule assessment at 18–19 months for ASD’s core characteristics.ResultsThe development of joint attention (JA) gestures was slower in the EL than in the LL group. The trajectories of the two groups began to diverge at 14–18 months. Children who reached the diagnostic cutoff point for ASD showed reductions in social interaction gestures at 12–13 months, in gestures integrated with any two communication skills (G-M) at 15–16 months; and in gestures integrated with eye contact (G-E) at 18–19 months. Overall gesture and G-M integration were associated with an overall ADOS communication and social interaction score.ConclusionsThe developmental trajectories of JA gestures of EL and LL children differed. G-M gestures represent early indicators that may be a predictor of ASD.

Ascribed or achieved? The role of birth order on innovative behaviour in the workplace

The pathway through which birth order triggers status competition and affects individuals’ subsequent innovative behaviours in the Chinese workplace remains an unexplored topic. To fill this lacuna, this study collected data from China to test the relationships. Findings showed that the first-born siblings tended to have a rational status striving orientation, which mediated the relationship between birth order and exploitative innovative behaviour; later-born siblings, though, were more likely to have an emotional status striving orientation, which weakened the relationship between birth order and exploratory innovative behaviour. Considering that expectations for the future career may affect an individual’s current decision, we also examined the moderating role of relative time distance and found that it strengthened the relationship between status striving orientation and employees’ innovative behaviour. Our study is of practical and theoretical significance by exploring the pathway through which birth order affects employees’ innovative behaviour in the Chinese context.

Intergenerational persistence of family formation trajectories among teenage-mothers and -fathers in Sweden

In this paper, we address the questions of whether early family trajectories of parents are reflected in childbearing teenagers, and how socio-economic and family background factors impact these intergenerational correlations. We use within-dyad sequence analysis to examine combined marital and childbearing trajectories, up to age 30, of two generations of a representative sample of childbearing teenagers born between 1975 and 1985 and their progenitors, drawn from the Swedish population register data. We find evidence for within-family persistence of early family trajectories, with better matches across family state sequences for dyads composed of childbearing teenagers and their parents, than for dyads composed of childbearing teenagers and parents of random birth cohort peers. Regression analysis shows that these intergenerational associations are stronger and occur among later-born siblings from non-traditional family backgrounds, and among families with lower socio-economic backgrounds. This study fills gaps in the knowledge of intergenerational family life course dynamics beyond the early parenthood event.

Learned Experience and Resource Dilution: Conceptualizing Sibling Influences on Parents' Feeding Practices.

Studies from diverse cultures report mixed results in the relationship between birth order and risk for obesity. Explanations may thus lie in the postnatal period when growth is shaped by the family environment, including parental feeding practices, which may be affected by siblings. Consistent with a family systems perspective, we describe two processes that may explain birth order effects on parental feeding practices and child outcomes: learned experience and resource dilution. Parents learn from experience when earlier-born children influence their parents’ knowledge, expectations, and behavior toward later-born siblings through their behaviors and characteristics—which can have both positive and negative implications. Resource dilution is a process whereby the birth of each child limits the time, attention and other resources parents have to devote to any one of their children. The goal of this review is to provide a theoretical basis for examining potential sibling influences on parental responsive feeding toward developing recommendations for future research and practice aimed at preventing obesity throughout family systems.

Does maternal grandmother's support improve maternal and child nutritional health outcomes? Evidence from Merida, Yucatan, Mexico.

In humans, high levels of investment are required to raise offspring, because of the prolonged developmental period and short interbirth intervals. The costs borne by individual mothers may be mitigated by obtaining social support from others. This strategy could be particularly valuable for first-time mothers, who lack first-hand experience and whose offspring have higher mortality risk than later-born siblings. As raising children is potentially stressful, mothers may gain from others sharing their experience, providing knowledge/information and emotional support. Being genetically related to both mother and grandchild, maternal grandmothers may be especially well placed to provide such support, while also gaining fitness benefits. We tested the over-arching hypothesis that first-time mothers and their young children supported by the maternal grandmother would have lower levels of stress and better health outcomes, compared to mother-infant dyads lacking such grandmaternal support. A cohort of 90 mother-infant dyads (52 with grandmaternal support, 38 without) was recruited in Merida, Mexico. We assessed anthropometry and body composition in both mother and child, along with maternally perceived stress and child temperament, and documented maternal social relationships. No differences were found in perceived stress/temperament or anthropometry of either mothers or children, according to the presence/absence of grandmaternal support. However, a composite score of whether grandmothers provided advice on infant feeding was positively associated with child nutritional status. Mothers without grandmaternal support reported seeking more informational and emotional support from other female relatives for childcare, potentially compensating for limited/absent grandmaternal support. Our findings may help develop interventions to improve maternal and child health by targeting the dynamics of maternal social networks. This article is part of the theme issue 'Multidisciplinary perspectives on social support and maternal-child health'.

NEONATAL DEATH IN INDIA: BIRTH ORDER IN A CONTEXT OF MATERNAL UNDERNUTRITION.

We document a novel fact about neonatal death, or death in the first month of life. Globally, neonatal mortality is disproportionately concentrated in India. We identify a large effect of birth order on neonatal mortality that is unique to India: later-born siblings have a steep survival advantage relative to the birth-order gradient in other developing countries. We show that India's high prevalence of maternal undernutrition and its correlation with age and childbearing can explain this pattern. We find that Indian mothers exit the underweight body mass range at an internationally comparatively high rate as they progress through childbearing careers.

New Partner, New Order? Multipartnered Fertility and Birth Order Effects on Educational Achievement

A substantial amount of research shows that younger siblings perform worse than their older sisters and brothers in several socioeconomic outcomes, including educational achievement. Most of these studies examined stable families and excluded half-siblings. However, the increasing prevalence of multipartnered fertility implies that many children grow up in nonnuclear families. We examine whether there is evidence for birth order effects in this context, which offers an opportunity to test and potentially expand the explanatory scope of the two main theories on birth order effects. We use comprehensive Norwegian registry data to study siblings in the 1985–1998 cohorts born to mothers or fathers who parented children with at least two partners. We provide evidence for negative effects of birth order on lower secondary school grades in both cases. Children born to fathers displaying multipartnered fertility tend to have lower grades than older full siblings but perform more similarly or better compared with older half-siblings. For siblings born to mothers with the multipartnered fertility pattern, later-born siblings do worse in school compared with all older siblings. This indicates that negative birth order effects tend to operate either within or across sets of full siblings, depending on the sex of the parent displaying multipartnered fertility. We argue that these findings can be explained by a combination of resource dilution/confluence theory and sex differences in residential arrangements following union dissolutions. We also suggest an alternative interpretation: maternal resources could be more important for generating negative birth order effects.

Dynamic Complementarity in Skill Production: Evidence From Genetic Endowments and Birth Order

The birth order literature emphasizes the role of parental investments in explaining why firstborns have higher human capital outcomes than their laterborn siblings. We use birth order as a proxy for investments and interact it with genetic endowments. Exploiting only within-family variation in both ensures they are exogenous as well as orthogonal to each other. As such, our setting is informative about the existence of dynamic complementarity in skill production. Our empirical analysis exploits data from 15,019 full siblings in the UK Biobank. We adopt a family-fixed effects strategy combined with instrumental variables to deal with endogeneity issues arising from omitted variables and measurement error. We find that birth order and genetic endowments interact: those with above-average genetic endowments benefit disproportionally more from being firstborn compared to those with below-average genetic endowments. This finding is a clean example of how genetic endowments ('nature’) and the environment ('nurture’) interact in producing educational attainment. Moreover, our results are consistent with the existence of dynamic complementarity in skill formation: additional parental investments associated with being firstborn are more ‘effective’ for those siblings who randomly inherited higher genetic endowments for educational attainment.

Having a Sibling With ADHD or ASD Increases Risk for Both Disorders

Back to table of contents Previous article Next article Clinical and Research NewsFull AccessHaving a Sibling With ADHD or ASD Increases Risk for Both DisordersNick ZagorskiNick ZagorskiSearch for more papers by this authorPublished Online:18 Jan 2019https://doi.org/10.1176/appi.pn.2019.1a4AbstractOverall, about 15 percent of later-born siblings in the ADHD and ASD risk groups were diagnosed with ADHD or ASD, compared with about 2 percent of children whose older siblings had neither disorder.Autism spectrum disorder (ASD) and attention-deficit/hyperactivity disorder (ADHD) are distinct childhood conditions, but they share many traits. Children with both disorders often display impulsivity, difficulties with social skills, and repetitive behaviors. A study published December 10, 2018, in JAMA Pediatrics identified a hereditary overlap as well. Children who have an older sibling who has been diagnosed with ASD are not only at an increased risk of ASD themselves, but also at an increased risk of developing ADHD. Similarly, children who have an older sibling diagnosed with ADHD are at an increased risk of being diagnosed with ADHD or ASD.iStock/romrodinka“[L]ater-born siblings of children with ASD and ADHD appear to be at elevated risk within and across diagnostic categories and thus should be monitored for both disorders,” wrote lead author Meghan Miller, Ph.D., an assistant professor of psychiatry and behavioral sciences at the University of California, Davis, and colleagues. “Practitioners may wish to share such information with families given the potential relevance of monitoring social communication, attention, and behavior regulation skills in later-born siblings of children with ASD or ADHD.”Miller and colleagues analyzed medical records from two large U.S. health care systems: Wisconsin’s Marshfield Clinic and Kaiser Pacific Northwest, which covers Oregon and Washington. They looked for families who had at least one more child after having a child diagnosed with ADHD or ASD. The authors did this to overcome the effect of “reproductive stoppage,” in which parents decide to stop having children after one is diagnosed with a developmental disorder. Family analyses that look only at the total number of siblings with or without a disorder likely underestimate the true inheritance risks, the authors wrote.The final dataset included 15,175 non-firstborn children; of these, 730 had an older sibling diagnosed with ADHD (the ADHD risk group) and 158 had an older sibling diagnosed with ASD (the ASD risk group).Compared with children whose older siblings had neither disorder, children in the ASD risk group were about 30 times more likely to also be diagnosed with ASD and 3.7 times more likely to be diagnosed with ADHD. Children in the ADHD risk group were about 13 times more likely to be diagnosed with ADHD and 4.4 times more likely to be diagnosed with ASD than children whose older siblings had neither disorder.Overall, about 15 percent of later-born siblings in the ADHD and ASD risk groups were diagnosed with ADHD or ASD, compared with about 2 percent of children whose older siblings had neither disorder. Miller told Psychiatric News that the analysis could not rule out that some of the increased prevalence of ADHD and/or ASD among the siblings in the ASD and ADHD risk groups was due to shared environmental factors. However, she said there is a lot of evidence that shows these two disorders are highly heritable.As expected, the odds of ASD or ADHD in later-born siblings increased even more if the child was a boy. In the ADHD risk group, the odds of an ADHD diagnosis in a later born sibling also increased if the older diagnosed sibling was a girl.“Some studies have suggested that there may be a ‘female protective effect’ for male-predominant disorders like ASD and ADHD,” Miller told Psychiatric News. That means females need to inherit a greater amount of genetic risk factors before they manifest a disorder. Therefore, if a daughter develops ADHD, it suggests the parents are carrying a greater genetic burden, which would make ADHD in boys more likely. The authors did not observe this same trend with older female siblings in the ASD risk group, which they suggested may have been due to the smaller number of diagnoses in the dataset.This study was supported by grants from the National Institute of Mental Health. ■ An abstract of “Sibling Recurrence Risk and Cross-Aggregation of Attention-Deficit/Hyperactivity Disorder and Autism Spectrum Disorder” can be accessed here. “Later Sibling Recurrence of Autism Spectrum Disorder and Attention-Deficit/Hyperactivity Disorder: Clinical and Mechanistic Insights,” here. ISSUES NewArchived

Birth Order, Parental Health Investment, and Health in Childhood

Research has shown that cognitive and non-cognitive skills, education and earnings decrease with birth order. Less is known about birth order effects on health. This paper provides a comprehensive analysis of the relationship between birth order, health at birth and in childhood, and parental health investment. High-quality administrative data on children born in Austria between 1984 and 2015 allow us to exploit within-family variation in birth order to account for confounding familylevel factors. In a sample of families with two to four children, we find statistically significant and quantitatively important birth order effects on health at birth and in primary school. These birth order effects are positive, in that later-born siblings are healthier than the first-born child, and increase with birth order. Consequently, first-born children are more likely to consume medical drugs and to utilize inpatient and outpatient medical services. We also find significant birth order differences in parental health investment. Compared to their later-born siblings, first-born children are more likely to receive preventive medical care and immunization against measles, mumps, and rubella.

Neonatal Death in India: Birth Order in a Context of Maternal Undernutrition

We document a novel fact about neonatal death, or death in the first month of life. Globally, neonatal mortality is disproportionately concentrated in India. We identify a large effect of birth order on neonatal mortality that is unique to India: later-born siblings have a steep survival advantage relative to the birth order gradient in other developing countries. We show that India's high prevalence of maternal undernutrition and its correlation with age and childbearing can explain this pattern. We find that Indian mothers exit the underweight body mass range at an internationally comparatively high rate as they progress through childbearing careers.

Sibling Recurrence Risk and Cross-aggregation of Attention-Deficit/Hyperactivity Disorder and Autism Spectrum Disorder

Attention-deficit/hyperactivity disorder (ADHD) and autism spectrum disorder (ASD) are believed to partially share genetic factors and biological influences. As the number of children with these diagnoses rises, so does the number of younger siblings at presumed risk for ADHD and ASD; reliable recurrence risk estimates within and across diagnoses may aid screening and early detection efforts and enhance understanding of potential shared causes. To examine within-diagnosis sibling recurrence risk and sibling cross-aggregation of ADHD and ASD among later-born siblings of children with either disorder. Using data extracted from medical records of 2 large health care systems in the United States, estimates of recurrence risk and cross-aggregation in later-born siblings of children with ADHD or ASD were compared with later-born siblings of children without these diagnoses. One data set included children seen between January 1, 1995, and December 31, 2013; the other included children born between January 1, 1998, and May 17, 2010. Participants included 15 175 later-born siblings of children with ADHD, ASD, and no known diagnosis. The study was conducted from October 2, 2017, to August 14, 2018. Diagnoses of ASD or ADHD in the later-born sibling, ascertained from medical records, were the primary outcomes of interest; moderators included sex, gestational age, and maternal age. A total of 15 175 later-born siblings were classified by familial risk status based on the older child's diagnostic status: ADHD risk (n = 730; male [51.92%]), ASD risk (n = 158; male [48.10%]), and no known risk (n = 14 287; male [50.73%]). Compared with later-born siblings of children without ADHD or ASD, later-born siblings of children with ASD were more likely to be diagnosed with ASD (odds ratio [OR], 30.38; 95% CI, 17.73-52.06) or ADHD in the absence of ASD (OR, 3.70; 95% CI, 1.67-8.21). Compared with later-born siblings of children without a diagnosis, later-born siblings of children with ADHD were more likely to be diagnosed with ADHD (OR, 13.05; 95% CI, 9.86-17.27) or ASD in the absence of ADHD (OR, 4.35; 95% CI, 2.43-7.79). Later-born siblings of children with ASD or ADHD appear to be at elevated risk for the same disorder, but also of being diagnosed with the other disorder. These findings provide further support for shared familial mechanisms underlying ASD and ADHD, which may be useful for genetic and prospective developmental studies. Later-born siblings of children with ADHD or ASD should be monitored for both conditions.

Diagnosis of Autism Spectrum Disorder After Age 5 in Children Evaluated Longitudinally Since Infancy

The diagnosis of autism spectrum disorder (ASD) has been found to be remarkably stable but few studies have followed children not initially diagnosed with ASD beyond 3 years of age to examine late or delayed diagnoses. The present study used a prospective familial-risk design to identify children who had undergone multiple comprehensive assessments in preschool and were determined to be negative for ASD only to meet criteria for ASD when tested in middle childhood. Data were pooled across 3 research teams studying later-born siblings of children with ASD. Fourteen children met inclusion criteria for the late-diagnosed group and were compared with a large sample of high- and low-risk siblings from the same sites who had ASD or typical development (TD) outcomes at 3 years of age. As a group, the late-diagnosed children scored between the TD and ASD groups on most measures administered at 3 years and differed significantly from the ASD group on most measures. However, there was significant heterogeneity among the late-diagnosed cases. Seven showed very little evidence of ASD in preschool, whereas 7 demonstrated subtle, subthreshold symptomatology. Some children with ASD might present with a subtle phenotype early in life or show a prolonged time course of symptom development. This emphasizes the need for screening and surveillance schedules that extend past 36 months and continued evaluation of any child who presents with atypical early development and/or high-risk status. The findings also shed light on reasons why the mean age of ASD diagnosis remains older than 4 years.

The birth order paradox: Sibling differences in educational attainment

This study uses population register data to examine the relationship between birth order and educational attainment in Sweden, and demonstrates that while the net effect of birth order on educational attainment is negative, later-born children often spend longer in education. The explanation for this finding is due to educational expansion in Sweden in the 20th century, which outweighs the negative causal effect of birth order for the affected cohorts. This is particularly true for women due to the fact that the rate of increasing educational enrolment has been greater for women than for men. These results also show that later-borns in large families particularly benefit from educational expansion due to the longer average birth interval between the first and last child in large families, meaning that the supply of educational opportunities increased to a greater extent in the intervening period. However, in periods where education is not expanding, later-born siblings continue to fare worse than first-borns.

Autism Spectrum Disorder Symptoms in Infants with Fragile X Syndrome: A Prospective Case Series

No studies to date have prospectively examined early autism spectrum disorder (ASD) markers in infants with fragile X syndrome (FXS), who are at elevated risk for ASD. This paper describes the developmental profiles of eight infants with FXS from 9 to 24 months of age. Four meet diagnostic criteria for ASD at 24 months of age, and four do not. Trends in these case studies suggest that early social-communicative deficits differentiate infants with and without later ASD diagnoses in ways that are similar to later-born siblings of children with ASD. Repetitive behaviors and cognitive and adaptive impairments are present in all FXS infants throughout development, suggesting that these deficits reflect the general FXS phenotype and not ASD in FXS specifically.

Birth order and preschool children's cooperative abilities: A within-family analysis.

There is evidence for a laterborn sibling advantage in some social skills, although this has not been investigated in children's early capacities for cooperation. Using a within-family design, this study compared firstborn and laterborn (i.e., middle and youngest) siblings on their cooperative abilities when they were aged around 3years. Further, the study investigated whether the association between children's birth order and cooperative abilities was dependent on the prosocial behaviour of other siblings in the home. The sample included 288 ethnically and sociodemographically diverse children clustered within 144 families. Cooperation was directly assessed using a problem-solving paradigm requiring two simultaneous and complementary actions of the child and adult tester to achieve a joint goal. Parents reported on the prosocial behaviour of up to four siblings in the home. Results of a multilevel analysis indicated that youngest children were more advanced in their cooperative abilities, compared to firstborn children, irrespective of their siblings' prosociality. Middle children, in contrast, were only advantaged over firstborn children if their siblings showed high levels of prosociality. The analysis accounted for a number of potential family-wide confounds, providing evidence that this is a child-specific effect related to birth order. Findings are discussed from a social constructivist perspective with an emphasis on the sibling relationship as a context for cooperative interactions that facilitate sociocognitive development. Statement of contribution What is already known on this subject? There are individual differences in children's early capacities for cooperation. Children's early cooperation has not been considered in relation to birth order and/or sibling interaction quality. What does this study add? Youngest children are advantaged in their cooperation as compared to firstborn children. Middle children are also advantaged, but only if their siblings are prosocial. The use of a developmentally sensitive within-family design reduces confounds common to birth order studies.

Parent Prediction of Autism Spectrum Disorder in Infants at Risk: A Follow-up Study

Later-born siblings of children with autism spectrum disorder are considered at biological risk for autism spectrum disorder and the broader phenotype. Early screening may detect first signs of autism spectrum disorder and expedite diagnosis and early intervention. In this follow-up study, we re-examine a validated second-degree autism spectrum disorder screener for children at biological risk of autism spectrum disorder, the parent observation of early markers scale. Using available follow-up information up to age 13 years, 110 children (the original 108 infants plus 2 infants recruited after the completion of the original study) were divided into three groups: diagnosed group (n = 13), lost diagnosis group (n = 5), and undiagnosed group (n = 92). The parent observation of early markers scale continued to show acceptable predictive validity. The parent observation of early markers scale total scores and mean number of elevated items were significantly higher in the diagnosed group than the undiagnosed group. The lost-diagnosis group did not differ from the undiagnosed group on parent observation of early markers scale total scores and elevated items at any age, but the lost-diagnosis group had significantly lower total scores, and number of elevated items than the diagnosed group starting at 18 months. Both autism spectrum disorder core and subsidiary behaviors differentiated the diagnosed and undiagnosed groups from 9 to 36 months of age. We hereby provide further evidence that the parent observation of early markers scale may serve as a low-cost early screener for autism spectrum disorder in at-risk children and pinpoint specific developmental ,and behavioral problems that may be amenable to very early intervention.

Gesture development in toddlers with an older sibling with autism.

Nonverbal communication deficits are characteristic of autism spectrum disorder (ASD) and have been reported in some later-born siblings of children with ASD (heightened-risk (HR) children). However, little work has investigated gesture as a function of language ability, which varies greatly in this population. This longitudinal study characterizes gesture in HR children and examines differences related to diagnostic outcome (ASD, language delay, no diagnosis) and age. We coded communicative gesture use for 29 HR children at ages 2 and 3 years during interactions with a caregiver at home. Children in the ASD group produced fewer gestures than their HR peers at 2 years, though large individual differences were observed within each subgroup at both ages. In addition, reliance on particular types of gestures varied with age and outcome. Both ASD and language delay children exhibited a pattern of reduced pointing relative to their no diagnosis peers. Similarities and differences exist between communication in HR infants with language delay and their HR peers, reinforcing our understanding of links between verbal and nonverbal communication in populations at risk for language delay.

Diagnostic stability in young children at risk for autism spectrum disorder: a baby siblings research consortium study.

The diagnosis of autism spectrum disorder (ASD) made before age 3 has been found to be remarkably stable in clinic- and community-ascertained samples. The stability of an ASD diagnosis in prospectively ascertained samples of infants at risk for ASD due to familial factors has not yet been studied, however. The American Academy of Pediatrics recommends intensive surveillance and screening for this high-risk group, which may afford earlier identification. Therefore, it is critical to understand the stability of an ASD diagnosis made before age 3 in young children at familial risk. Data were pooled across seven sites of the Baby Siblings Research Consortium. Evaluations of 418 later-born siblings of children with ASD were conducted at 18, 24, and 36 months of age and a clinical diagnosis of ASD or Not ASD was made at each age. The stability of an ASD diagnosis at 18 months was 93% and at 24 months was 82%. There were relatively few children diagnosed with ASD at 18 or 24 months whose diagnosis was not confirmed at 36 months. There were, however, many children with ASD outcomes at 36 months who had not yet been diagnosed at 18 months (63%) or 24 months (41%). The stability of an ASD diagnosis in this familial-risk sample was high at both 18 and 24 months of age and comparable with previous data from clinic- and community-ascertained samples. However, almost half of the children with ASD outcomes were not identified as being on the spectrum at 24 months and did not receive an ASD diagnosis until 36 months. Thus, longitudinal follow-up is critical for children with early signs of social-communication difficulties, even if they do not meet diagnostic criteria at initial assessment. A public health implication of these data is that screening for ASD may need to be repeated multiple times in the first years of life. These data also suggest that there is a period of early development in which ASD features unfold and emerge but have not yet reached levels supportive of a diagnosis.

Familial recurrence of autism spectrum disorder: evaluating genetic and environmental contributions.

This study was designed to examine the pattern of familial recurrence of autism spectrum disorder (ASD) in terms of genetic and environmental contributions related to timing of birth. The authors linked California Department of Developmental Services records with state birth certificates to identify all siblings and half siblings of individuals affected with ASD born between 1990 and 2003. A total of 6,616 full siblings, 644 maternal half siblings, and 299 paternal half siblings born after ASD index cases were used to calculate recurrence risks. Control families, identified through matching to cases, were included for comparison (a total of 29,384 siblings). The overall sibling recurrence risk was 10.1%, compared with a prevalence of 0.52% in siblings of controls. The recurrence risk in second-born children was higher (11.5%) than in later-born siblings (7.3%); a similar pattern was observed for maternal half siblings (6.5% for second-born compared with 3.0% for later-born siblings; 4.8% overall). The recurrence risk was significantly higher for siblings who immediately followed the index case in birth order compared with those later in birth order. The recurrence risk for paternal half siblings (2.3%) was half the overall recurrence risk for maternal half siblings but was similar to that for later-born maternal half siblings. An exponential effect of short interbirth interval was observed, with the recurrence risk reaching 14.4% for an interbirth interval of 18 months or less, compared with 6.8% for an interval of 4 years or more. An identical phenomenon was observed in maternal half siblings. The results support genetic susceptibility in the familial recurrence of ASD along with factors related to timing of birth.