Via Chris Blattman, Stefano della Vigna and Alec Kennedy find that “movie reviewers are mildly biased”:http://elsa.berkeley.edu/~sdellavi/wp/movieratings11-09-07.pdf when they review movies made by studios affiliated to their own parent corporation.
bq. we know of no systematic evidence on distortions in coverage induced by cross-holdings. We provide systematic and quantitative evidence in a particular setting. We focus on two groups–News Corp. and Time-Warner–and measure how media outlets in these groups review movies distributed by an aﬃliate in the group– 20th Century Fox and Warner Bros. Pictures, respectively. The advantage of focusing on movie reviews is that they are frequent, easily quantiﬁable, and are believed to inﬂuence movie attendance in the early weeks of release (Reinstein and Snyder, 2005), with clear monetary beneﬁts to the studio distributing the movie. … We compare the review of, say, Avatar (distributed by 20th Century Fox) by the Wall Street Journal to the reviews by other outlets not owned by News Corp. Since the Wall Street Journal reviewer may have a diﬀerent evaluation scale from other reviewers, we use as a further control group the reviews of movies distributed by a diﬀerent studio, such as Paramount. If the Wall Street Journal provides systematically more positive reviews for 20th Century Fox movies, but not for Paramount movies, we conclude thatconﬂict of interest induces a bias. …
bq. For the media outlets owned by News Corp., in the favorite empirical speciﬁcation we ﬁnd that these media outlets give a more positive review to the 20th Century Fox movies by 2.3 points out of 100. The eﬀect is relatively small, the equivalent of raising the review score by one star (on a zero-to-four scale) for one out of twelve movies. … For the media outlets owned by Time Warner, we ﬁnd no evidence of bias due to crossholdings. The ﬁnding of no bias is not due to lack of power, since we can reject any bias in the reviews larger than 0.7 out of 100 points. In fact, we reject the hypothesis that the bias due to conﬂict of interest is the same for the two conglomerates. … Interestingly, we ﬁnd no consistent evidence of bias by omission for the News Corp. outlets, but we ﬁnd evidence for two of the Time Warner outlets: CNN.com and Time magazine. Both of these media are signiﬁcantly more likely to review movies which other reviewers rated highly, when the movie is distributed by an aﬃliated studio. … Altogether, this evidence suggests that bias by omission and bias by commission may be substitutes, rather than complements.
NB that the results are preliminary, the authors ask readers not to cite them etc (I don’t think that a blogpost counts as a cite …).