Loneliness and the onset of depression in old age are growing problems related to the greater life expectancy nowadays. This review investigated the longitudinal association between loneliness and depressive symptoms in the elderly.

A comprehensive search was conducted using three databases (Scopus, PsycInfo, and PubMed) combing for empirical studies published up until July 2020. A total of 4.549 abstracts and 221 full-text articles were assessed. Three authors independently reviewed titles and abstracts; disagreements were resolved by consensus.

Ten studies were included in the final review. We identified two categories of studies based on the outcome considered in each article: 1) the longitudinal effect of loneliness on depressive symptoms and 2) the clinical course of depression and its association with loneliness. All the articles reported a significant and positive association between loneliness and depressive symptoms in their longitudinal design research, ranging from an odds ratio of 0.41 to 17.76. The heterogeneity regarding the effect size in the analyses can be explained by the multifactorial design implemented by most of the studies included.

Future research should investigate the moderators' role and how it may influence the longitudinal association between loneliness and depression over the years.