The lucid interval is a long-held legal concept widely accepted in case law as a possible means of countering a challenge to testamentary and related capacities. In parallel, the clinical phenomenon of cognitive fluctuations has been considered a common element of several neurodegenerative disorders (dementias), including Alzheimer Disease, but is especially prevalent in vascular dementia and dementia with Lewy bodies. In this article, we review the objective evidence for cognitive fluctuations in dementia and the implications for the validity of the legal notion of the lucid interval cited in recent case law. The literature on cognitive fluctuations in dementia shows that such fluctuations largely affect attention and alertness, rather than memory or the higher level executive functions that are essential components of testamentary capacity. Moreover, these fluctuations are small in magnitude and very short in duration. These findings cast doubt on the validity of the lucid interval and invite a critical rethinking of this legal concept as applied to will challenges involving testators with dementia.
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