Causal discovery and the problem of psychological interventions

Abstract

Finding causes is a central goal in psychological research. In this paper, I argue based on the interventionist approach to causal discovery that the search for psychological causes faces great obstacles. Psychological interventions are likely to be fat-handed: they change several variables simultaneously, and it is not known to what extent such interventions give leverage for causal inference. Moreover, due to problems of measurement, the degree to which an intervention was fat-handed, or more generally, what the intervention in fact did, is difficult to reliably estimate. A further complication is that the causal findings in psychology are typically made at the population level, and such findings do not allow inferences to individual-level causal relationships. I also discuss the implications of these problems for research, as well as various ways of addressing them, such as focusing more on the discovery of robust but non-causal patterns.

Introduction

A key objective in psychological research is to distinguish causal relationships from mere correlations (Kendler & Campbell, 2009; Pearl, 2009; Pearl & Mackenzie, 2018; Shadish & Sullivan, 2012). For example, psychologists want to know whether having negative thoughts is a cause of anxiety instead of just being correlated with it: If the relationship is causal, then the two are not just spuriously hanging together, and intervening on negative thinking is one way of reducing anxiety in patients suffering from anxiety disorders. However, to what extent is it actually possible to find psychological causes?

In this paper, I will point out and discuss several obstacles to discovering psychological causes that have not been adequately discussed in the psychological and philosophical literature. First, I will highlight a crucial but often neglected distinction: The distinction between non-psychological and psychological interventions, which create very different contexts for causal inference. Second, I will argue that the latter context, discovery of psychological causes, is deeply problematic.

I will formulate my arguments in the framework of the interventionist theory of causation. It is a theory of causation that aims at elucidating the role of causal thinking in science, and defining a notion of causation that captures the difference between causal relationships and mere correlations in a way that reflects scientific practice (Pearl, 2000, 2009; Spirtes, Glymour, & Scheines, 2000; Woodward, 2003; Woodward & Hitchcock, 2003). Several authors have also argued that interventionism adequately captures the role of causal thinking and reasoning in psychological research (Campbell, 2007; Kendler & Campbell, 2009; Rescorla, 2018; Woodward, 2008a). Due to its conceptual clarity regarding the notions of causation and intervention, interventionism is exceptionally well suited for highlighting the problems that are the focus of this paper. However, as I will argue, these problems are not just restricted to interventionism, but should be seen as general challenges to the discovery of psychological causes.

The structure of this paper is as follows. I will start by giving a brief introduction to interventionism, and then turn to problems of interventionist causal inference in psychology: First, to problems related to psychological interventions (section 3), and then to problems arising from the requirement to “hold fixed” possible confounders (section 4). After this, I will show how these problems appear in some of the most common approaches to causal discovery in psychological research (section 5). In the last section, I consider different ways forward and various implications that my arguments have for psychology.