To the Editor: We appreciate that Dr. Hall and colleagues draw attention to the problem of defining the concepts of "spirituality" and "religion." Several multidimensional or pluralistic definition systems of religion have indeed been proposed over the years. However, none of them offers a perfect solution to the need of operational tools in research on a possible biological underpinning of religious and spiritual behavior.

The authors criticize our article for using the terms "religion" and "spirituality" interchangeably. However, we did not use the concept "religion" in the article. Rather, the concept "religious behavior," which is operationally defined in the literature (1), denotes cognitive and emotional behavior associated with (the individual’s relationship to their) religious beliefs. The term "spirituality" has been used in a wider context, including internal, subjective experiences, and has not been consistently defined by operational criteria. It is worth noting that the concept of spirituality is not necessarily linked to organized religion.

"Religious behavior" and "spirituality" are both covered by the personality subscale of the Spiritual Acceptance Scale, which was used in our study. The Spiritual Acceptance Scale consists of 13 items that include cognitive affirmation and values as well as subjective experiences of mystical quality. Thus, the definition of religion at a sociocultural level, as suggested by Dr. Hall and colleagues, is not covered by the scale used in our study and belongs to a different discussion.

Another part of Dr. Hall and colleagues’ criticism is their interpretation that aspects of mystical experiences can be mediated by the central serotonin system. We do not suggest that the serotonin system per se mediates mystical experiences but instead may act as a sensory filter (2). Low serotonin 5-HT_1A receptor binding potential may be associated with a low filter function, thus paving the way for sensory stimuli otherwise not experienced. The more narrow focus on mystical experiences in this part of the discussion in our article (pp. 1967–1968) was given by comparisons made with pharmacological mechanisms causing similar experiences in man.

Finally, we agree with Dr. Hall and colleagues that it would be interesting to repeat this study in different populations. Epidemiological studies provide support for the view that religious behavior (in a more narrow sense) and spirituality (in a wider sense) are influenced by both genetic and environmental factors (3, 4). Given the previously demonstrated genetic contribution to religious behavior and spirituality, it is a promising strategy to use interindividual variability in neuroreceptor binding as a tool to approach the multifaceted question of why people vary in spiritual zeal and also within the same religious belief system.