The aim of the research was to investigate whether high levels of the stress hormone cortisol interfere with verbal declarative memory.

All participants were employees or students at the Washington University Medical Center. All participants were given a clinical interview with a physician. They were excluded from the sample if they were pregnant, had a history of mental illness, had suffered head trauma, or had suffered from an illness that had been treated with corticosteroids.

The researchers carried out a double blind laboratory experiment in which the participants were matched for age and gender to one of three conditions:

• Condition 1 – high level of cortisol: The participants in the high-level cortisol group were given a tablet containing 160 mg of cortisol on each day of the four-day experiment. This dose of cortisol produces blood levels similar to those seen in people experiencing a major stress event.

• Condition 2 – low level of cortisol: The participants in the low level of cortisol group were given a tablet containing 40 mg of cortisol per day. This dose is similar to the amount of cortisol circulating in the bloodstream of people undergoing minor surgical procedures such as having stitches removed.

• Condition 3 – placebo group: The participants in this condition were given placebo tablets - that is, a tablet that looked like the other tablets but with no active ingredient. This was done in order to have a control group.

All participants were asked to listen to and recall a prose paragraph. Each day they were given a different piece of prose with the same level of difficulty.  They were tested three times. 

All participants were first tested before taking any cortisol.  The finding was that there was no significant difference between groups. This is an important control to make sure that at baseline, individual differences will not be a confounding variable for the study.

The participants were tested again one day after taking the pill and then again four days later.  There was a test again six days later to make sure that there were no long-term effects of the treatment on the participants.

The results indicated that high cortisol levels impaired performance in the memory task since the participants who received the highest level of cortisol also showed the worst performance in verbal declarative memory. The researchers were also able to verify that the effect was not permanent; the performance of participants in the high cortisol condition returned to normal after they stopped taking the hormone tablet. According to the researchers, these results demonstrate a clear link between levels of cortisol and remembering.  It appears that high levels of cortisol interfered with the recall of the prose passage, whereas moderate levels of cortisol may have actually assisted in the recall of the passage. This makes sense as there are cortisol receptor sites on the hippocampus, which is responsible for the transfer of information from STM to LTM and vice versa.

• Since this study was experimental, the researchers could establish a clear cause and effect relationship between the IV and the DV.
• The researchers carried out a baseline test in order to eliminate the possible confounding variable of individual differences between groups.
• The experiment ran over several days and the participants were not in the lab the whole time, so the researchers did not have full control over extraneous variables - for example, individual stressors in the lives of the participants. In spite of this, there was a clear relationship between the amount of cortisol ingested and the performance on the memory test.
• Different texts were used for days 1 and 4.  However, the researchers counterbalanced the use of the texts to act as a control that the difficulty of the text did not play a role in recall.
• Memorizing a piece of prose is perhaps not the most authentic memory experience. Although it may explain student exam stress, the ability to generalize the results to other situations may be limited.
• There are ethical considerations in the study. The participants ingested cortisol which affected their memory negatively. However, the participants had signed an informed consent form and the damage was not permanent.