against intruders. This causes a rise in aggression and testosterone levels in the defenders. A similar effect has been observed in football with testosterone levels being significantly higher in home games than in away games. Goalkeepers, the last line of defense, have particularly strong testosterone changes when playing against a bitter rival as compared to a training season. How testosterone may influence results is unclear but may include cognitive effects such as motivation and physiological effects such as reaction time. An extreme example of home advantage was the 2013 Nigeria Premier League; each of the 20 teams lost at most 3 of 19 home matches and won at most 3 of 19 away matches. Paul Doyle ascribed this to visiting teams' facing "violent crowds, questionable refereeing and [...] [a]rriving just before kick-off after long road trips, often on hazardous surfaces". The 2020–21 NHL season saw major disruption due to COVID-19-restricted conditions that resulted in bubble playoffs and ghost games, as fans were unable to attend in person. New research has shown that this led to a significant drop to home advantage compared with the previous six seasons. In 592 games played under the restricted conditions through March, home teams suffered a decline of 10% while road teams’ win rates increased by 7%. Factors related to the game rules In a number of sports, the hosting team has the advantage of playing with their first choice uniforms, while the visiting team wears their alternative away/road colors. Some sports leagues simply state that the team wears its away uniforms only when its primary jerseys would clash with the colors of the home team, while other leagues mandate that visiting teams must always wear their away colors regardless. However, sometimes teams wear their alternative uniforms by choice. This is especially true in North American sports where generally one uniform is white or grey, and "Color vs. color" games (e.g., blue vs. red uniforms) are a rarity, having been discouraged in the era of black-and-white television. Many teams from warm-weather cities may wear their white uniforms at home, forcing their opponent to wear dark uniforms in the hot weather. An exception is a rule in high school American football (except Texas, which plays by NCAA football rules), requiring the home team to wear dark jerseys and the visiting team to wear white jerseys, which may work as a disadvantage to the home team in