Gullibility may be a warning sign of Alzheimer's disease dementia.

ABSTRACTBackground:Theory of Mind reasoning, which plays a pivotal role in social interaction, is required to detect deception. Empirically, those with cognitive decline are vulnerable to deception.

METHODS: Participants were 45 healthy elderly adults with clinical dementia rating (CDR) 0, and 76 outpatients: 25 with amnestic mild cognitive impairment (aMCI, CDR 0.5), 34 with mild Alzheimer's disease dementia (ADD, CDR 1), and 17 with moderate ADD (CDR 2). The task consisted of two single-frame cartoons that depicted a character with an intention to deceive another character using social signs of gaze and pointing, and participants are provided clue questions to detect the character's intentions.

RESULTS: The percentage of participants who detected the character's intention decreased with ADD progression (CDR 0, 82.2%; CDR 0.5, 48.0%; CDR 1, 29.4%; and CDR 2, 0%). Total score (0-6) also decreased with ADD progression (CDR 0, 4.4 +/-1.1; CDR 0.5, 3.0 +/-1.3; CDR 1, 2.9 +/-1.5; and CDR 2, 1.6 +/-0.9).

DISCUSSION AND CONCLUSIONS: The present study demonstrated that those with aMCI have difficulty in detecting other's deceiving intentions, when the intention was shown implicitly using social signs. In a previous study, we have reported that mild ADD showed difficulties in detecting intention, while aMCI succeeded in detection when the intention was depicted explicitly. These results together suggested that those with aMCI is vulnerable to deception when the intention was shown implicitly using non-verbal cues, while ADD may fail to detect the intention even when the intention was shown explicitly.