Evolving scientific evidence is reshaping how courts assess the reliability of expert testimony in product liability and pharmaceutical litigation across the United States.
Plaintiffs’ experts frequently rely on epidemiologic studies to support opinions on general causation, affected populations, latency, and damages, often assuming that population patterns remain static over time.
As diagnostic practices change, data sources expand, and longer follow-up periods alter how disease patterns are observed, the conclusions drawn from epidemiologic studies may shift considerably.
A recent study on autism spectrum disorder highlights how quickly the scientific landscape can move beneath the feet of litigants and their expert witnesses.
The CDC estimates that 1 in 31 eight-year-old children in the United States has ASD based on 2022 data, compared with 1 in 150 in 2000 based on data from six sites.
A population-based study by Fyfe et al., examining more than 2.7 million individuals born in Sweden between 1985 and 2020, illustrates how epidemiologic understanding of ASD continues to evolve.
The CDC reports a male-to-female diagnosis ratio of 3.4 to 1 based on 2022 data, but Fyfe et al. found that this widely cited male predominance narrows significantly with increasing age at diagnosis and over calendar time.
By age 20 in the final follow-up year of 2022, the cumulative male-to-female ratio had narrowed to 1.2 to 1, a finding that underscores how observed population trends may reflect diagnostic timing rather than biology alone.
Autism-related claims often involve alleged exposures occurring years before any diagnosis is made, and Fyfe et al.’s data show that such gaps are common, particularly among females.
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 actually support.
Federal courts have already demonstrated willingness to exclude expert testimony where causation opinions cannot be reliably linked to the available literature, as seen in the Southern District of New York’s ruling in In re Acetaminophen – ASD-ADHD Products Liability Litigation.
Under Federal Rule of Evidence 702 and the Daubert standard, courts are expected to scrutinise whether expert opinions rest on a methodology that is both reliable and appropriately applied to the facts at hand.
Similar evidentiary challenges arise in cases involving environmental exposures, drugs and medical devices, and chronic diseases where diagnosis may occur long after the alleged exposure event.
For defence counsel, the core lesson is straightforward: when epidemiology changes, expert evidence must be evaluated with close attention to context, methodology, and what the underlying data actually demonstrates.
Expert opinions are only as reliable as the evidentiary foundation on which they rest, both in terms of completeness and the currency of the science being applied.
A planned third instalment in this series will focus on how judges should evaluate scientific literature when assessing the reliability of expert testimony under Daubert.