Following up on Part I, we now look at a recent empirical study whose discussion provides what I consider a particularly thoughtful exposition on causal inference with longitudinal survey data.
As someone who studies adolescent drinking and how it may be affected by parental socialization, I took notice of an article by Stephanie Madon and colleagues entitled “Self-Fulfilling Prophecy Effects of Mothers’ Beliefs on Children’s Alcohol Use…” (Journal of Personality and Social Psychology, 2006, vol. 90, pp. 911-926).
The substantive findings of longitudinal relations between mothers’ beliefs at earlier waves and children’s drinking at later waves were interesting. However, what really stood out to me was the thoughtful discussion near the end of the article about which criteria for causality the longitudinal correlational design could and could not satisfy, and what other lines of argument could be marshaled on behalf of causal inferences. Within the larger Discussion section, Madon and colleagues’ consideration of causal inference appears under the heading “Interpreting Results From Naturalistic Studies.”
Madon et al.’s discussion notes the major thing their design does: “…rule out the possibility that the dependent variable exerted a causal influence on the predictor variable because measurement of the predictor variable is temporally antecedent to changes in the dependent variable.” It also acknowledges the major limitation of the design, possible unmeasured third variables: “…the potential omission of a valid predictor raises the possibility that mothers based their beliefs on a valid predictor of adolescent alcohol use that was not included in the model. If this were to occur,… [mothers’] self-fulfilling effects would be smaller than reported.”
The authors then proffer “several reasons why we believe that the self-fulfilling prophecy interpretation is more compelling” than a third-variable account.
First, Madon and colleagues claim, their preferred interpretation is consistent with established experimental findings of self-fulfilling prophecies. “Although this convergence does not prove that our results reflect self-fulfilling prophecies, confidence in the validity of a general conclusion increases when naturalistic and experimental studies yield parallel findings.”
Second, the authors cite the breadth of “theoretically and empirically supported predictors” they used as control variables. This step, they claim, reduces the chance of spurious, third-variable causation.
Third, the authors note that maternal beliefs exerted greater predictive power over time, whereas the non-maternal predictors explained less variance over time. The extension of this argument, as I understand it, would then be to ask, How likely is it that any as-yet-unmeasured, potential third variable tracks perfectly with maternal beliefs and not at all with the predictors that served as control variables?
Both by their empirical findings and their citation of the broader adolescent drinking (in terms of their control variables) and self-fulfilling prophecy (for its experimental foundation) literatures, Madon and colleagues provide a great deal of evidence that is consistent with a parenting effect.
Depending on one’s perspective, to use a football analogy, one might conclude that the Madon et al. study has advanced the ball to the opponents’ 40-yard line, or 20-yard line, or even 1-yard line. But most would probably agree that it takes a true experiment to get the ball into the end zone.