title: Who and what is a "population"? Historical debates, current controversies, and implications for understanding "population health" and rectifying health inequities.
abstract: BACKGROUND
  The idea of "population" is core to the population sciences but is rarely defined except in statistical terms. Yet who and what defines and makes a population has everything to do with whether population means are meaningful or meaningless, with profound implications for work on population health and health inequities.
  
  
  METHODS
  In this article, I review the current conventional definitions of, and historical debates over, the meaning(s) of "population," trace back the contemporary emphasis on populations as statistical rather than substantive entities to Adolphe Quetelet's powerful astronomical metaphor, conceived in the 1830s, of l'homme moyen (the average man), and argue for an alternative definition of populations as relational beings. As informed by the ecosocial theory of disease distribution, I then analyze several case examples to explore the utility of critical population-informed thinking for research, knowledge, and policy involving population health and health inequities.
  
  
  RESULTS
  Four propositions emerge: (1) the meaningfulness of means depends on how meaningfully the populations are defined in relation to the inherent intrinsic and extrinsic dynamic generative relationships by which they are constituted; (2) structured chance drives population distributions of health and entails conceptualizing health and disease, including biomarkers, as embodied phenotype and health inequities as historically contingent; (3) persons included in population health research are study participants, and the casual equation of this term with "study population" should be avoided; and (4) the conventional cleavage of "internal validity" and "generalizability" is misleading, since a meaningful choice of study participants must be in relation to the range of exposures experienced (or not) in the real-world societies, that is, meaningful populations, of which they are a part.
  
  
  CONCLUSIONS
  To improve conceptual clarity, causal inference, and action to promote health equity, population sciences need to expand and deepen their theorizing about who and what makes populations and their means.
authors.affiliation: Department of Society, Human Development and Health, Harvard School of Public Health, Boston, MA 02115, USA. nkrieger@hsph.harvard.edu
authors.forename: Nancy
authors.initials: N
authors.lastname: Krieger
mesh_terms: D019317:Evidence-Based Medicine; D054624:Health Status Disparities; D049672:History, 19th Century; D049673:History, 20th Century; D049674:History, 21st Century; D006801:Humans; D011153:Population; D011157:Population Dynamics; D011159:Population Surveillance; D011634:Public Health / Q000266:history; D015980:Public Health Practice; D009626:Terminology as Topic