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★★☆Single study — solid evidence, such as one good trial or a large observational dataset, but not replicated at the top tier.
In one trial, a single dose of cortisol dulled recall for facts
Forty healthy adults took 10 mg of cortisol. An hour later they recalled facts and handled space worse than people on placebo. Memory for how to do things was untouched.
What the study found
Facts slipped, habits did not, an hour after 10mg of cortisol
Cortisol itself does this, not just the situation that raises it. Healthy adults given a single 10mg dose of cortisol scored worse an hour later at recalling facts and at spatial thinking than adults given a placebo, while memory for how to do things was untouched.
The caveat · A single dose, not months of it
Forty people took the dose, and each took it once, so this shows what one sharp rise does rather than what months of raised cortisol do. The loss was confined to recalling facts and handling space, which leaves much of what memory does unaffected.