Psychologists in many fields face a dilemma. Whereas most researchers are aware that randomized experiments are considered the "gold standard" for causal inference, manipulation of the independent variable of interest will often be unfeasible, unethical, or simply impossible. One can hardly assign couples to stay married or get a divorce; nonetheless, one might be interested in the causal effect of divorce on well-being. One cannot randomly resettle individuals into different strata of society, but one might be concerned about the causal effects of social class on behavior. One cannot randomize children to different levels of adversity, yet one might care about the potential negative consequences of childhood adversity on health in adulthood. This article provides very general guidelines for researchers who are interested in any of the many research questions that require causal inferences to be made on the basis of observational data. Researchers from different areas of psychology have chosen different strategies to cope with the weaknesses of observational data. To circumvent the issue altogether, some researchers have implemented "surrogate interventions": If the real-life cause of interest cannot be manipulated, there might be a proxy that can be randomized in the lab. For example, an influential study on the effects of social class on prosocial behavior included an experimental manipulation of perceived social class. Participants were asked to compare themselves with either the top or the bottom of the "social ladder," so as to temporarily change their subjective 745629A MPXXX10.

Replication—an important, uncommon, and misunderstood practice—is gaining appreciation in psychology. Achieving replicability is important for making research progress. If findings are not replicable, then prediction and theory development are stifled. If findings are replicable, then interrogation of their meaning and validity can advance knowledge. Assessing replicability can be productive for generating and testing hypotheses by actively confronting current understandings to identify weaknesses and spur innovation. For psychology, the 2010s might be characterized as a decade of active confrontation. Systematic and multi-site replication projects assessed current understandings and observed surprising failures to replicate many published findings. Replication efforts highlighted sociocultural challenges such as disincentives to conduct replications and a tendency to frame replication as a personal attack rather than a healthy scientific practice, and they raised awareness that replication contributes to self-correction. Nevertheless, innovation in doing and understanding replication and its cousins, reproducibility and robustness, has positioned psychology to improve research practices and accelerate progress. Expected final online publication date for the Annual Review of Psychology, Volume 73 is January 2022. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

Causal inference is a central goal of research. However, most psychologists refrain from explicitly addressing causal research questions and avoid drawing causal inference on the basis of nonexperimental evidence. We argue that this taboo against causal inference in nonexperimental psychology impairs study design and data analysis, holds back cumulative research, leads to a disconnect between original findings and how they are interpreted in subsequent work, and limits the relevance of nonexperimental psychology for policymaking. At the same time, the taboo does not prevent researchers from interpreting findings as causal effects—the inference is simply made implicitly, and assumptions remain unarticulated. Thus, we recommend that nonexperimental psychologists begin to talk openly about causal assumptions and causal effects. Only then can researchers take advantage of recent methodological advances in causal reasoning and analysis and develop a solid understanding of the underlying causal mechanisms that can inform future research, theory, and policymakers.