Thinking About My Baby: Does Paternity Leave Affect Performance? by Jeremy T. Raff July 13, 2021 As an ardent follower of the Baltimore Orioles, I’ve experienced a lot of bad baseball over the past few years, and one specific bit of bad baseball caught my eye recently. On April 5th, Shawn Armstrong returned from the paternity list after his wife gave birth just a few days before. He was continually demolished over the next week, giving up six earned runs in two innings of relief. He wasn’t getting too unlucky either, even if his FIP (20.12) was below his ERA (27.00). As the parent of a three-year-old, I thought back to my first week after work following a month-long paternity leave. I was distracted, tired, and couldn’t wait to get home at the end of the day. Of course Armstrong got lit up, he just became a dad a few days before! Maybe professional athletes aren’t staying up all night changing diapers, but it stills seems like they would perform worse after a trip to the paternity list as they reorient their life. Is that true though? Do athletes perform worse after returning from the paternity list? Fortunately, Baseball Prospectus tracks all paternity leave going back to MLB’s implementation of the policy in 2011. Instead of parsing through every season of data, I just focused on the most recent ones: 2017-2020. This still provided 115 different trips to the paternity leave list, enough to give an idea of trends and differences. I separated these individuals into pitchers (62) and hitters (53) to make for easier comparison. For hitters, I used wRC+ as my key metric and tracked it across 7-day, 14-day, and full season time frames. For pitchers, I used ERA and FIP and tracked those across the same time frames. There are a few quick caveats. Occasionally players will make an appearance before promptly being demoted or ending up on the injured list. I’ve kept the same number for the 7- and 14-day span even if the player didn’t make an appearance during that time frame. This only accounts for six players (five pitchers and a hitter), a fairly small amount of the sample. Pitchers Returning From Paternity Leave Time Frame ERA Mean Added ERA to Total FIP Mean Added FIP to Total 7 Days 5.7604021 .09293 4.706660804 .07591 14 Days 4.667755914 .07529 4.446168986 .071712 Full Season 3.922911761 .06327 3.99527033 .063349 The impact on hitters isn’t quite as noticeable, without any clear trend. Hitters’ wRC+ is actually higher within seven days of a paternity list visit compared to their full season performance. There is an 8-point gap between performance 14 days after a paternity list visit when compared to the full season numbers, but it’s hard to see how this squares with the 7-day performance. None of the differences are statistically significant at the 95% confidence interval. Hitters Returning From Paternity Leave Time Frame wRC+ Mean Added wRC+ to Total 7 Days 98.58 1.86 14 Days 90.86 1.71 Full Season 96.78 1.83 In summary, a trip to the paternity list doesn’t seem to have much of an impact for players. Maybe Shawn Armstrong was pitching badly just because he’s a bad pitcher; There’s a reason he’s since been DFA’d and passed through waivers. The performance for pitchers does still pique my interest, as there is a consistent trend when looking at 7-day, 14-day, and full-season performance across ERA and FIP. Despite this, the only statistically significant difference is between 7-day ERA and full-season ERA, far from anything conclusive. The small sample (263.2 innings) and other confounding variables leave it far from conclusive in any direction for pitchers, especially given the other t-test results. It does appear, however, interesting enough to look at in a larger sample. A future quantitative analysis incorporating additional years of data may be able to provide more comprehensive answers. It may also be an area where qualitative research can provide answers on the impact of pitcher preparation, stamina, and overall performance.