Katherine MacLean

Katherine MacLean was a psychologist on the Johns Hopkins psilocybin research team who studied the lasting effects of psilocybin experiences and guided dozens of sessions. Her work linked participants’ reports of profound experiences to measurable changes in personality, while her own experience as a guide unsettled her earlier atheism. In her account, the research mattered not only for what it showed about participants, but also for what close attention to their journeys did to the people who accompanied them.

Research and lasting change

MacLean analyzed survey data from 52 volunteers who had taken part in the first groups of healthy participants studied at Johns Hopkins. The follow-up included interviews with friends and family members whom the volunteers had designated, adding observations from people who knew them outside the research setting.

The results pointed to lasting changes among participants who met the study’s threshold for a complete mystical experience, as measured by the Mystical Experience Questionnaire. Along with sustained improvements in well-being, these volunteers showed long-term increases in openness to experience, one of the five traits psychologists use to assess personality. The trait includes aesthetic sensitivity, imagination, and tolerance of other people’s viewpoints and values; it is also associated with creativity in the arts and sciences.

MacLean emphasized the unusual durability of the finding: pronounced, lasting changes in the personalities of adults are rare. The follow-up thus gave empirical weight to accounts of personal transformation, while tying the reported change to a particular kind of experience rather than treating every psilocybin session as equivalent.

The guide’s perspective

MacLean was not only an analyst of follow-up data. During her time at Johns Hopkins, she guided dozens of psilocybin sessions, occupying the role of sitter alongside participants as they underwent experiences that could alter their outlook. Her work placed her close to the personal material that surveys and interviews later attempted to measure.

She described a gradual shift in her own worldview. Starting from an atheist position, she said, she repeatedly encountered things in her work that did not sit easily with that belief; as she sat with people on psilocybin, her world came to seem more mysterious. The change is not presented as a formal conclusion of the research, but as a consequence of sustained exposure to other people’s experiences.

Significance

MacLean’s role brings two kinds of evidence into view: the follow-up study’s measured association between complete mystical experiences and enduring openness, and a guide’s account of how witnessing such experiences affected her own understanding. The distinction matters. Her report does not establish that the sessions proved a supernatural claim; it shows that the work could complicate a guide’s prior assumptions even as researchers sought to describe its effects in psychological terms.

Together, her research and testimony make MacLean a link between the study’s statistical findings and the human encounters behind them. Participants’ changes were assessed through questionnaires and follow-up conversations, but the sessions also changed the perspective of someone whose task was to remain present with them.