  Decision support is the science and associated practice that consist in
providing recommendations to decision makers facing problems, based on
available theoretical knowledge and empirical data. Although this activity is
often seen as being concerned with solving mathematical problems and conceiving
algorithms, it is essentially an empirical and socially framed activity, where
interactions between clients and analysts, and between them and concerned third
parties, play a crucial role. Since the 80s, two concepts have structured the
literature devoted to analysing this aspect of decision support: validity and
legitimacy. Whereas validity is focused on the interactions between the client
and the analyst, legitimacy refers to the broader picture: the organisational
context, the overall problem situation, the environment, culture, history.
Despite its importance, this concept has not received the attention it deserves
in the literature in decision support. The present paper aims at filling this
gap. For that purpose, we review the literature in other disciplines relevant
to elaborate a concept of legitimacy useful in decision support contexts. Based
on this review, we propose a general theory of legitimacy, adapted to decision
support contexts, encompassing the relevant contributions we found in the
literature. According to this general theory, a legitimate decision support
intervention is one for which the decision support provider produces a
justification that satisfies two conditions: (i) it effectively convinces the
decision support provider's interlocutors (effectiveness condition) and (ii) it
is organised around the active elicitation of as many and as diverse
counterarguments as possible (truthfulness condition). Despite its conceptual
simplicity, legitimacy, understood in this sense, is a very exacting
requirement, opening ambitious research avenues that we delineate.
