Background:
Loneliness—a mismatch between desired and existing socialrelationships—has been linked to an increased dementia risk, but the nature ofthis association remains unclear and may involve reverse causation. Personality traits,such as higher neuroticism and lower extraversion, are also associated with bothdementia risk and loneliness. Few studies, however, have accounted for personalitywhen examining the relationship between loneliness and cognitive aging. This studyaims to clarify the association between midlife loneliness and late-life cognition byconsidering the potential moderating role of personality, using a long follow-up up to23 years.

Method:
Participants from the older Finnish Twin Cohort completed questionnaires in1975, 1981, and 1990. Current feeling of loneliness was measured in all questionnaireswith one item with three response options (very lonely, fairly lonely, not lonely).Neuroticism and extraversion were based on an abbreviated Eysenck’s PersonalityInventory in 1975 and 1981. Covariates included sex, age, education, CardiovascularRisk Factors, Aging and Dementia score, alcohol use, smoking status, depression status,social contact frequency, and marital status. Cognitive data were collected in 2013–2017 from 1814 individuals aged 71–79 (mean age 34 in 1975) using the TelephoneInterview for Cognitive Status–modified (TICS-m). ApoE status was determined fromDNA arrays.

Result:
Participants were grouped as consistently lonely (n =202), never lonely (n=1238), or with varying loneliness (n =127). Loneliness profile and extraversion didnot predict cognition, after adjustment for covariates. Higher neuroticism predictedworse cognition when adjusting for age, sex, and education (β = -0.127, p = 0.045),one-point increase in neuroticism was linked to a 0.13-point decrease in TICS-m (0–50), but this was not significant after adjusting for other covariates. No significantinteraction effects were found between loneliness profile, personality traits, and cognition. Covariates age, education, depression status, social contact frequency, andApoE status consistently predicted cognitive scores.

Conclusion:
​​​​​​​Midlife loneliness was not associated with late-life cognitive functioning.While neuroticism showed a modest link to lower cognition, this did not persist afterincluding all covariates. These findings suggest that loneliness and personality inmidlife may not be independent risk factors for late-life cognitive decline when broaderhealth and lifestyle factors are considered.