When two competing sports teams are similar in rank more fouls occur during a match according to research published in Biology Letters. The findings suggest that human conflicts escalate more when groups are more similar in competitive ability.
Using data from two professional sports (football and basketball), Gert Stulp and his team at the University of Groningen in the Netherlands, found that in both sports aggression increased when the difference in rank was smaller. This has only previously been shown to be the case in individual, rather than group, conflicts.
The researchers looked at data from 1530 football matches from five seasons of the highest professional German football division and 1230 games from a season of NBA basketball. Fouls such as illegal physical contact towards other players (such as unfair tackles) were included in the data, and other illegal actions that are penalized by the referee (such as offside positioning in handball) were not included. Thus the number of fouls incurred is a good indicator of aggression during the match.
When there was no difference in rank between the football (or basketball) teams, 39.15 (or 42.36) fouls were estimated to be committed, as opposed to the average 38.76 (or 41.71) fouls. And when the highest-ranked team played the lowest-ranked team only 34.59 (40.56) fouls were estimated to be committed.
A proximate explanation for these findings is provided by Neave & Wolfson, who showed that testosterone in football players was higher before playing against self-perceived extreme rivals compared with playing against a perceived moderate rival. Thus, playing against a well-matched team may well increase the testosterone and hence the aggressiveness of the players, which results in more fouls.