Kayuet Liu

Social Influence and the Autism Epidemic

Despite a plethora of studies, we do not know why autism incidence has increased rapidly over the past two decades. Using California data, this study shows that children living very close to a child previously diagnosed with autism are more likely to be diagnosed with autism. An underlying social influence mechanism involving information diffusion drives this result, contributing to 16% of the increase in prevalence over 2000–2005. We eliminate competing explanations (i.e., residential sorting, environmental toxicants, and viral transmission) through seven tests and show that information diffusion simultaneously contributed to the increased prevalence, spatial clustering, and decreasing age of diagnosis.

Closely Spaced Pregnancies Are Associated With Increased Odds of Autism in California Sibling Births

OBJECTIVE: To determine whether the interpregnancy interval (IPI) is associated with the risk of autism in subsequent births. METHODS: Pairs of first- and second-born singleton full siblings were identified from all California births that occurred from 1992 to 2002 using birth records, and autism diagnoses were identified by using linked records of the California Department of Developmental Services. IPI was calculated as the time interval between birth dates minus the gestational age of the second sibling. In the primary analysis, logistic regression models were used to determine whether odds of autism in second-born children varied according to IPI. To address potential confounding by unmeasured family-level factors, a case-sibling control analysis determined whether affected sibling (first versus second) varied with IPI. RESULTS: An inverse association between IPI and odds of autism among 662 730 second-born children was observed. In particular, IPIs of <12, 12 to 23, and 24 to 35 months were associated with odds ratios (95% confidence intervals) for autism of 3.39 (3.00–3.82), 1.86 (1.65–2.10), and 1.26 (1.10–1.45) relative to IPIs of ≥36 months. The association was not mediated by preterm birth or low birth weight and persisted across categories of sociodemographic characteristics, with some attenuation in the oldest and youngest parents. Second-born children were at increased risk of autism relative to their firstborn siblings only in pairs with short IPIs. CONCLUSIONS: These results suggest that children born after shorter intervals between pregnancies are at increased risk of developing autism; the highest risk was associated with pregnancies spaced <1 year apart.

Cohort effects explain the increase in autism diagnosis among children born from 1992 to 2003 in California

BACKGROUND The incidence and prevalence of autism have dramatically increased over the last 20 years. Decomposition of autism incidence rates into age, period and cohort effects disentangle underlying domains of causal factors linked to time trends. We estimate an age-period-cohort effect model for autism diagnostic incidence overall and by level of functioning. METHODS Data are drawn from sequential cohorts of all 6 501 262 individuals born in California from 1992 to 2003. Autism diagnoses from 1994 to 2005 were ascertained from the California Department of Development Services Client Development and Evaluation Report. RESULTS Compared with those born in 1992, each successively younger cohort has significantly higher odds of an autism diagnosis than the previous cohort, controlling for age and period effects. For example, individuals born in 2003 have 16.6 times the odds of an autism diagnosis compared with those born in 1992 [95% confidence interval (CI) 7.8-35.3]. The cohort effect observed in these data is stronger for high than for low-functioning children with an autism diagnosis. DISCUSSION Autism incidence in California exhibits a robust and linear positive cohort effect that is stronger among high-functioning children with an autism diagnosis. This finding indicates that the primary drivers of the increases in autism diagnoses must be factors that: (i) have increased linearly year-to-year; (ii) aggregate in birth cohorts; and (iii) are stronger among children with higher levels of functioning.

Social demographic change and autism

Parental age at child’s birth—which has increased for U.S. children in the 1992–2000 birth cohorts—is strongly associated with an increased risk of autism. By turning a social demographic lens on the historical patterning of concordance among twin pairs, we identify a central mechanism for this association: de novo mutations, which are deletions, insertions, and duplications of DNA in the germ cells that are not present in the parents ’ DNA. Along the way, we show that a demographic eye on the rising prevalence of autism leads to three major discoveries. First, the estimated heritability of autism has been dramatically overstated. Second, heritability estimates can change over remarkably short periods of time because of increases in germ cell mutations. Third, social demographic change can yield genetic changes that, at the population level, combine to contribute to the increased prevalence of autism.

Suicide rates in the world: 1950-2004

The cross-country differences and the trends of suicide rates in 71 countries from 1950 to 2004 are described. The data are from the World Health Organizations Mortality Database. It shows that suicide rates vary greatly across countries, even within the same region or at similar levels of development. Random-effect models were used to examine the between-country and within-country stabilities in suicide rates. The results show that more than 90% of the variance in suicide rates is due to between-country differences, suggesting suicide rates display a strong temporal stability.

THE SPATIAL STRUCTURE OF AUTISM IN CALIFORNIA, 1993–2001

This article identifies significant high-risk clusters of autism based on residence at birth in California for children born from 1993 to 2001. These clusters are geographically stable. Children born in a primary cluster are at four times greater risk for autism than children living in other parts of the state. This is comparable to the difference between males and females and twice the risk estimated for maternal age over 40. In every year roughly 3% of the new caseload of autism in California arises from the primary cluster we identify-a small zone 20 km by 50 km. We identify a set of secondary clusters that support the existence of the primary clusters. The identification of robust spatial clusters indicates that autism does not arise from a global treatment and indicates that important drivers of increased autism prevalence are located at the local level.

Interaction Domains and Suicide: A Population-based Panel Study of Suicides in Stockholm, 1991–1999

This article examines how suicides influence suicide risks of others within two interaction domains: the family and the workplace. A distinction is made between dyad-based social-interaction effects and degree-based exposure effects. A unique database including all individuals who ever lived in Stockholm during the 1990s is analyzed. For about 5.6 years on average, 1.2 million individuals are observed, and 1,116 of them commit suicide. Controlling for other risk factors, men exposed to a suicide in the family (at work) are 8.3 (3.5) times more likely to commit suicide than non-exposed men. The social-interaction effect thus is larger within the family domain; yet work-domain exposure is more important for the suicide rate because individuals are more often exposed to suicides of coworkers than family members.

Spatial clusters of autism births and diagnoses point to contextual drivers of increased prevalence

Autism prevalence has risen dramatically over the past two decades in California. Although often suggested to have been crucial to the rise of autism, environmental and social contextual drivers of diagnosis have not been extensively examined. Identifying the spatial patterning of autism cases at birth and at diagnosis can help clarify which contextual drivers are affecting autisms rising prevalence. Children with autism not co-morbid with mental retardation served by the California Department of Developmental Services during the period 1992-2005 were matched to Californias Birth Master Files. We search for spatial clusters of autism at time of birth and at time of diagnosis using a spatial scan approach that controls for key individual-level risk factors. We then test whether indicators of neighborhood-level diagnostic resources are associated with the diagnostic clusters and assess the extent of clustering by autism symptom severity through a multivariate scan. Finally, we test whether children who move into neighborhoods with higher levels of resources are more likely to receive an autism diagnosis relative to those who do not move with regard to resources. Significant birth and diagnostic clusters of autism are observed independent of key individual-level risk factors. While the clusters overlap, there is a strong positive association between the diagnostic clusters and neighborhood-level diagnostic resources. In addition, children with autism who are higher functioning are more likely to be diagnosed within a cluster than children with autism who are lower functioning. Most importantly, children who move into a neighborhood with more diagnostic resources than their previous residence are more likely to subsequently receive an autism diagnosis than children whose neighborhood resources do not change. We identify birth and diagnostic clusters of autism in California that are independent of individual-level autism risk factors. Our findings implicate a causal relationship between neighborhood-level diagnostic resources and spatial patterns of autism incidence but do not rule out the possibility that environmental toxicants have also contributed to autism risk.

Psychiatric history modifies the gender ratio of suicide: an East and West comparison

ObjectiveGender ratios of suicide rates differ greatly across countries. Victoria has a high male:female ratio in suicide that is typical in English-speaking and European countries, while in Hong Kong the low ratio is similar to other SouthEast Asian countries. This study investigates the effect of gender in the psychiatric and non-psychiatric populations to examine how psychiatric history may modulate the effect of gender in these two different communities.MethodReview of coronial documentation of all suicide cases in 2000 in Hong Kong and Victoria, Australia. Log-linear model was used to test the gender-psychiatric history-location interaction.ResultsGender difference in suicide was narrower in victims with psychiatric history than those without in both communities, albeit gender remained to have an effect even among those with a psychiatric history in Victoria. The impact of cultural- and gender-specific factors is most apparent in suicide cases with no prior psychiatric history in Victoria: the male:female ratio was as high as 8:1 in this group of victims. Log-linear model results show that the gender-psychiatric history-location interaction was statistically significant.ConclusionsThe gender differentials in suicide rates in these two communities are mainly driven by gender’s effect in the population with no psychiatric history. Severe clinical conditions can override some, but not all, of gender’s effects in Victoria. Suicide prevention effort should target gender-specific factors to prevent suicides in men without history of psychiatric disorders.

The Disappearing Seasonality of Autism Conceptions in California

Background Autism incidence and prevalence have increased dramatically in the last two decades. The autism caseload in California increased 600% between 1992 and 2006, yet there is little consensus as to the cause. Studying the seasonality of conceptions of children later diagnosed with autism may yield clues to potential etiological drivers. Objective To assess if the conceptions of children later diagnosed with autism cluster temporally in a systematic manner and whether any pattern of temporal clustering persists over time. Method We searched for seasonality in conceptions of children later diagnosed with autism by applying a one-dimensional scan statistic with adaptive temporal windows on case and control population data from California for 1992 through 2000. We tested for potential confounding effects from known risk factors using logistic regression models. Results There is a consistent but decreasing seasonal pattern in the risk of conceiving a child later diagnosed with autism in November for the first half of the study period. Temporal clustering of autism conceptions is not an artifact of composition with respect to known risk factors for autism such as socio-economic status. Conclusion There is some evidence of seasonality in the risk of conceiving a child later diagnosed with autism. Searches for environmental factors related to autism should allow for the possibility of risk factors or etiological drivers that are seasonally patterned and that appear and remain salient for a discrete number of years.