Science Doesn’t Stand Still: Evolving Evidence and Expert Reliability Under Daubert

This is the second part in a “Law Meets Science” series on how the scientific record affects expert evidence in litigation. Part I examined the implications of unpublished and selectively reported research. This alert focuses on a separate but equally important issue: how the evolution of scientific knowledge can affect the reliability of expert opinions over time.

Evolving epidemiology can affect scientific understanding and the reliability of expert testimony. Plaintiffs’ experts often rely on epidemiologic evidence to support opinions on general causation, affected populations, latency, and damages, sometimes assuming those population patterns are static. As diagnostic practices change, data sources expand, and longer follow-up periods alter how disease patterns are observed, epidemiologic conclusions may also shift.

A recent study on autism spectrum disorder (ASD) highlights this potential shift. The prevalence of ASD continues to rise in the U.S. and globally, due at least in part to changes in diagnostic practices and surveillance. ASD has become a focal point in pharmaceutical and product liability litigation. Changes in ASD epidemiology can therefore be highly relevant to the reliability of expert opinions and an effective litigation strategy.

Using 2022 data from U.S. sites, the CDC estimates that 1 in 31 eight-year-old children has ASD, compared with 1 in 150 in 2000 based on data from six sites. A recent population-based study by Fyfe et al. (2026), examining more than 2.7 million individuals born in Sweden between 1985 and 2020, illustrates how epidemiologic understanding of ASD continues to evolve. Historically, ASD has been diagnosed more often in males than in females. Based on 2022 data, for example, the CDC reports a male-to-female ratio of 3.4 to 1. Fyfe et al., however, found that the widely cited male predominance narrows with increasing age at diagnosis and over calendar time. Although males were diagnosed more often in childhood, females continued to be diagnosed later, often in adolescence and beyond. By age 20 in the final follow-up year, 2022, the cumulative male-to-female ratio had narrowed to 1.2 to 1. This finding underscores that observed population trends may reflect diagnostic timing and case identification, not biology alone.

With guardrails (such as Federal Rule of Evidence 702 and Daubert) in place to help ensure expert testimony is reliable and relevant, these findings may matter in litigation. Epidemiologic studies measure observed outcomes, not necessarily disease onset. Experts who rely on diagnosis-based studies without accounting for diagnostic practices, surveillance, follow up time, population characteristics, or latency may overstate what the data can support.

Evolving diagnostic timelines can complicate simplified latency narratives. Autism-related claims often involve alleged exposures years before diagnosis. Fyfe et al.’s data show that such gaps are common, particularly among females, and that later diagnosis does not necessarily mean later disease onset. Experts should consider other possible causes, factors that may influence the data, and the difference between underlying traits and the timing of diagnosis.

In recent autism-related litigation, federal courts have excluded expert testimony when plaintiffs could not reliably link their experts’ causation opinions to the literature. The Southern District of New York’s ruling in In re Acetaminophen – ASD-ADHD Products Liability Litigation is an example of the careful judicial review courts should perform when expert opinions rely on evolving or incomplete science.

Fyfe et al. does not change the basic science of autism causation, but it does show that epidemiologic findings may be shaped by diagnostic practices, follow-up time, and outcome definitions that evolve over time. Similar issues arise in cases involving environmental exposures, drugs and medical devices, and chronic diseases in which diagnosis may occur long after the alleged exposure. For defense counsel, the takeaway is clear: when epidemiology changes, expert evidence should be evaluated with close attention to context, methodology, and what the evidence actually shows. Under Daubert and Rule 702, that scrutiny remains essential.

Taken together with Part 1, these issues underscore a common principle: expert opinions are only as reliable as the evidentiary foundation on which they rest, both in terms of completeness and currency. Careful scrutiny of what the scientific record includes, omits, and reveals over time remains central to effective Daubert challenges and defensible expert analysis. Originally planned as a two-part series, this series will now include a third installment focused on a deeper dive into how judges should evaluate scientific literature when assessing expert reliability.