Loneliness Across Phylogeny and a Call for Comparative Studies and Animal Models

Abstract

Loneliness typically refers to the feelings of distress and dysphoria resulting from a discrepancy between a person’s desired and achieved levels of social relations, and there is now considerable evidence that loneliness is a risk factor for poor psychological and physical health. Loneliness has traditionally been conceptualized as a uniquely human phenomenon. However, over millions of years of evolution, efficient and manifold neural, hormonal, and molecular mechanisms have evolved for promoting companionship and mutual protection/assistance and for organizing adaptive responses when there is a significant discrepancy between the preferred and realized levels of social connection. We review evidence suggesting that loneliness is not a uniquely human phenomenon, but, instead, as a scientific construct, it represents a generally adaptive predisposition that can be found across phylogeny. Central to this argument is the premise that the brain is the key organ of social connections and processes. Comparative studies and animal models, particularly when integrated with human studies, have much to contribute to the understanding of loneliness and its underlying principles, mechanisms, consequences, and potential treatments.

Myeloid differentiation architecture of leukocyte transcriptome dynamics in perceived social isolation

“…PSI represents a discrepancy between an animal's preferred and actual social relations and triggers phylogenetically conserved neuroendocrine responses (3)(4)(5). In humans, PSI involves an implicit hypervigilance for social threat (4)(5)(6).…”

“…PSI represents a discrepancy between an animal's preferred and actual social relations and triggers phylogenetically conserved neuroendocrine responses (3)(4)(5). In humans, PSI involves an implicit hypervigilance for social threat (4)(5)(6). In animal models, threat-related signaling from the sympathetic nervous system (SNS) to the bone marrow hematopoietic niche can stimulate the development of immature, glucocorticoid-resistant monocytes and neutrophils (myeloid lineage immune cells) via β-adrenergic up-regulation of the myelopoietic growth factor, granulocyte-macrophage colony-stimulating factor (GM-CSF) (7,8).…”

Pathways from Ageism to Loneliness

“…Similarly, the evolutionary theory of loneliness (Cacioppo et al 2015a) views loneliness as an aversive signal deriving from a discrepancy between desired and actual social relationships, which motivates people to socially engage and reconnect. According to this perspective, loneliness plays a key role in the evolution of humanity, because reconnection with others is crucial for survival and reproduction (Goossens et al 2015).…”

Loneliness Effects on Personality: Preprint

“…Human beings are social beings and are hence equipped with a desire for social inclusion (Baumeister & Leary, 1995;Cacioppo et al, 2015). If this desire is unfulfilled or threatened, most individuals react by reestablishing social ties and reaffiliating with others (Qualter et al, 2015) to end their state of loneliness.…”