Catherine Saint Louis, "Dessert, Laid-Back and Legal", NYT 5/14/2011:
Remember melatonin? In the 1990s, this over-the-counter dietary supplement was all the rage among frequent fliers, promoted as the miracle cure for jet lag. Now it is back in vogue, this time as a prominent ingredient in at least a half-dozen baked goods that flagrantly mimic the soothing effects of hash brownies — and do so legally. At least for now. […]
“A hangover effect has been reported” with large doses, said Anna Rouse Dulaney, a toxicologist with the Carolinas Poison Center. But she added, “I don’t want to go on the record saying this drug ‘can’ cause respiratory issues, that should be a ‘may.’ ”
Dhananjay Jagannathan, who sent in this example, wondered whether "evidentiality and modality have gotten a bit muddled" here, and suggest that perhaps the distinction between can and may has become biomedical jargon for the difference between an established statistical association ("this drug can cause respiratory issues", i.e. there's good evidence that it sometimes does) and a hypothesis suggested by some scattered or anecdotal evidence ("this drug may cause respiratory issues", i.e. there are reasons to think that it sometimes might).
There are 2820 results for "X can cause Y", and the first 10 all involve cases where a published study or other authoritative pronouncement claims an association to exist between X and Y: "Henna tattoos can cause severe skin reactions"; "New roads can cause congestion"; "Oral sex can cause throat cancer"; "Trains generate pressure waves in tunnels that can cause passengers' ears to pop"; "New roads can cause congestion"; "Malaria drug can cause mental disorder"; "Electric shocks can cause motor neurone disease"; "Female genital mutilation can cause infertility"; "Lack of sleep can cause brain damage and affect memory"; "Exposure to pesticides can cause Parkinson's"; "Submicroscopic defects in chromosomes can cause mental handicap".
There are 718 results for "X may cause Y", but these seem to be more variable. Some are certainly examples of hypothetical, weakly-supported or contested connections: "Space debris may cause mysterious ball lightning"; "Lead may cause mystery male infertility"; "Weedkiller may cause amphibian loss"; "Inflammation may cause cerebral palsy"; "Marijuana may cause pregnancies to fail"; "Falling stars may cause rainy days"; "Increased CO2 may cause plant life to raise rivers"; "Expanding waistlines may cause shrinking brains"; "Bioweapons alerts may cause lasting psychological harm"; "Brain defect may cause 50% of cot deaths".
But in other cases, may in the headline does not seem to be associated with a weaker connection. Thus the story under the headline "iPods may cause pacemakers to miss a beat" starts by expressing the same idea with can:
iPods can cause implantable cardiac pacemakers to malfunction, according to a study presented at a meeting of heart specialists on Thursday.
And the evidence supporting the connection seems to have been at least as strong as in many of the "can cause" stories:
The study tested the effect of placing the portable device near the chest of 100 patients with an average age of 76 – all were fitted with pacemakers. Technicians monitored information from an electrocardiogram and from the pacemaker itself, via a telemetry screen, during the trials.
Some form of interference was detected half of the time when an iPod was held 5 centimetres from the patient's chest for between 5 and 10 seconds.
A story running under the headline "Inflammation may cause deadly blood clots" likewise presents evidence at least as good as the evidence in many of the 'X can cause Y' stories:
People who are suffering from a severe infection are more likely to develop dangerous blood clots, a new study suggests. Researchers say that patients with respiratory and urinary tract infections have a temporarily doubled risk of deep vein thrombosis (DVT), a potentially deadly condition in which blood clots block circulation in large veins.
A distinction in evidentiary strength between "can cause" and "may cause" is presumably due to selection of one of the many meanings of can (glossed by the OED as "to have the power, ability or capacity") and one of the many meanings of may (glossed as "Expressing present subjective possibility, i.e. the admissibility of a supposition"). In English and other natural languages, evidentiality has been "getting a bit muddled" with other aspects of modality such as ability and permission for a long time. What seems to be new here is the emergence, in certain contexts, of an association of can and may with graded degrees of empirical support and epistemic strength.
The cited examples are not meant to imply that reports in the New Scientist are always credible evaluations of evidentiary status — see "Envy, Navy, Whatever" (Language Log 10/27/2006), "A Plea to Save New Scientist" (The n-Category Café 9/19/2006), or Blake Stacey's discussion of the EmDrive case at Contrary Brin, for discussion of a couple of five-year-old cases in detail. For some slightly more recent examples, see "Language and personality", LL 6/28/2008; "Pop platonism and unrepresentative samples", LL 7/26/2008;