Twelve Eyes See More than Eight. Referee Bias and the Introduction of Additional Assistant Referees in Soccer

23 Pages Posted: 21 May 2019

See all articles by Olivier Verstraeten

Olivier Verstraeten

Ghent University

Stijn Baert

Ghent University; University of Antwerp; Catholic University of Louvain (UCL); Institute for the Study of Labor (IZA)


We are the first to investigate whether the introduction of additional assistant referees in the UEFA Europa League (in season 2009-2010) and the UEFA Champions League (in season 2010-2011) was associated with lower referee bias. To this end, we analyse a unique database with pre- and within-game characteristics of all games in seven recent seasons in these leagues by means of bivariate probit regression models. We find evidence for substantial referee bias before the introduction of additional referees, while no such evidence is found after the introduction. Furthermore, additional assistants go hand in hand with more yellow cards for both home and away teams. We show that these findings are robust to multiple operationalisations of referee bias and that they are not just picking up a general time evolution towards less referee bias or the effect of parallel reforms.

Keywords: Soccer, referee bias, assistant referees, bivariate probit model, UEFA Champions League, UEFA Europa League

JEL Classification: L83, J44, Z00

Suggested Citation

Verstraeten, Olivier and Baert, Stijn, Twelve Eyes See More than Eight. Referee Bias and the Introduction of Additional Assistant Referees in Soccer. IZA Discussion Paper No. 12142. Available at SSRN:

Olivier Verstraeten

Ghent University

Coupure Links 653
Gent, 9000

Stijn Baert (Contact Author)

Ghent University ( email )

University of Antwerp ( email )

Prinsstraat 13
Antwerp, Antwerp 2000

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Catholic University of Louvain (UCL) ( email )

Place Montesquieu, 3
Louvain-la-Neuve, 1348

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Institute for the Study of Labor (IZA) ( email )

P.O. Box 7240
Bonn, D-53072


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