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I love sports. As a lifelong sports fan, I am all too familiar with the "home-field advantage." Home teams tend to win more than away teams. This is evidenced in betting lines, and in any cursory view of win/loss percentages. Without actually doing the analyses, I would be willing to bet this applies to every pro sports league all over the world where there is a team traveling to another team's stadium in a different part of the country.
Since age 14 or so, I have wondered how much of this is due to various factors related to playing at your home field (I was clearly the cool kid in school). When you play at home, you benefit from the roar of the crowd backing you. This seems to be the most common reason—just based on my recollection of being a sports fan—given for this advantage. As a social psychologist, I like this explanation. People impacting people in a tangible way to do better (or worse if you are the away team!). That reads like a scoreboard for why my field matters. Go ... err ... social psychology team?
But, there are loads of other potential benefits to playing at home. Often the dimensions of the field of play change when at home vs. away. Experience in that setting matters. You also have less travel time, and are closer to friends and families. You just, broadly, know where things are and how to navigate life more when playing at your home field.
I had a hunch that leagues would frown upon my requests to play without fans, hence my prior thirst for this answer went unquenched. With COVID-19, though, I can compare records with and without fans being physically present in the stadiums. Silver linings in a sea of suffering and death, I guess.
I am a busy man, though not as busy as I would like to convince my wife I am when the dishes need done. As such, I decided to only pick one league for this analysis. I have long followed the English Premier League (again, I was a big hit in rural Ohio growing up), and they just so happen to be playing without fans while still traveling to different stadiums. Destiny really.
{I initially was going to do baseball, so here is a gif of the time Randy Johnson hit a bird with a fastball. I didn't want to not use it. He would later use those talents in a deodorant commercial where his dodgeball throwing power represented odor fighting. Amazing really, a but I need to get back on topic.}
Luckily for me soccerstats.com has the away/home win percent for every team in the premier league dating back years. Teams in this league can either win (3 points), lose (0 points), or tie (1 point). Last season was interrupted due to COVID-19. I decided to take the previous five years of stats pre-COVID, with crowds, and compare it to all results post COVID-19, including all of last year and the beginning of this season.
Interestingly, the home team won 45.6% of the time pre-COVID from 2014-2018, with the away team winning 30.04% of the time. That's a clear home-field advantage with fans in the seats. Post COVID, the win percentage for home teams has been 44.4% (sample size 100 + games), and the away team has won 31.8% of the time.
With fans in seats, home teams are only winning 1.2% more games than without fans in seats. That leaves a large percentage of the home-field advantage to be explained by other factors that in my experience/recollection tend to get much less attention. It suggests being familiar with the stadium, sleeping in more typical surroundings, not having to travel, etc, might play a larger role than having a crowd physically there.
Last thing: A note of caution is that the vast majority of the post-COVID data is end of last season, and I did not check if home/away win percentages differ across the timepoint of a season. Also, it is very possible that the crowd makes more of a difference in leagues where stadiums have the same playing field. In the premier league, fields differ in size across stadiums which could possibly make it more of a factor relative to having a crowd.
