Scanning method and results.

<p>This figure is illustrative; for the precise time periods of the clusters and relative risks, see <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0041265#pone-0041265-t002" target="_blank">Table 2</a>.</p

Distributions of gestational ages of children with autism and of all births and the odds ratios of sole autism by gestational age.

<p>Distributions of gestational ages of children with autism and of all births and the odds ratios of sole autism by gestational age.</p

Descriptive statistics on the study population, N (%)^a.

a<p>Percentages are relative to all live births. Categories not shown (<12 years of education and Non-Hispanic white) are the reference categories in our analyses. Their comparable statistics can be calculated easily from the table.</p>b<p>Data for the year 2000 (shown here and as used in the analyses) consist of children conceived in January through March.</p

Temporal clusters of conceptions of children later diagnosed with autism from three year scans.

<p>Temporal clusters of conceptions of children later diagnosed with autism from three year scans.</p

Summary of criteria for a valid sibling sex ratio test.

<p>Summary of criteria for a valid sibling sex ratio test.</p

Percentages of siblings of cases and of controls conceived within timeframes of <i>t</i> days of conception of the respective case or control child, independent of the year of conception.

<p>Percentages of siblings of cases and of controls conceived within timeframes of <i>t</i> days of conception of the respective case or control child, independent of the year of conception.</p

Odds ratios of being A) born preterm and B) born with a low birth weight for those conceived within a temporal cluster.^d.

<p>CI: 95% Confidence Interval.</p>d<p>Odds ratios are relative to conceptions outside of the cluster but within the time period window <b><i>W</i></b> in which the cluster is found. For example, the reference category for those in the 1994 conception cluster is those who were conceived outside of the cluster but within window <b><i>W</i></b> 1992/3/1 to 1995/3/1. Significant results are in bold.</p

Odds ratios of being conceived within a temporal cluster.^c.

<p>CI: 95% Confidence Intervals.</p>c<p>Odds ratios are relative to all conceptions within the time period window <b><i>W</i></b> in which the cluster is found. For example, odds ratios for children conceived in the 1994 cluster are relative to those conceived in the window <b><i>W</i></b> 1992/3/1 to 1995/3/1. Significant results are in bold.</p

Possible causal structures underlying a selection bias induced association between autism and sibling sex.

<p>Preferences regarding sex composition may affect future fertility decisions, and hence observation (O) of additional siblings. This is represented here by arrows from sexes of child 1 and 2, S₁ and S₂, to O₃, the observation (or not) of a third child. If either autism in previous children (A₁ and A₂), or shared factors associated with autism (U; ex. parental age at the start of childbearing) also affect future fertility, selection bias may result. This is due to <i>de facto</i> conditioning on O₃ (only children who are conceived and born may be observed), opening a blocked path between autism and child sex. Conditioning on the sex of prior children will block these paths. A box drawn around a variable represents conditioning on it, which, unless it is a collider, blocks any paths through it.</p

Possible causal structures underlying a hypothesized association between autism and sibling sex.

<p>In the common cause scenario (A), an association arises because risk of autism and sex of each child share a common antecedent (for example, maternal testosterone levels). In the sex independence scenarios (B, C), risk of autism in siblings shares a common cause (B), or observation of autism in one sibling directly influences observation of autism in the other (C), but sex is independent. In the separate causes scenario (D), both sex of siblings within a given family and risk of autism in those children are correlated, but the association arises through separate antecedent factors. In the autism independence scenario (E), sex of siblings shares a common cause but risk of autism does not. Depending on method of analysis, an association between whether autism is diagnosed in one child (A₁) and sex of the sibling (S₂) could potentially be observed under any of these scenarios.</p