updated 09/01/2008 AT 7:00 PM ET
•originally published 09/02/2008 AT 6:55 AM ET
“If my dad has anything to say about it, I’m sure he will be a Cubs fan.” the Windy City native told PEOPLE Sunday at the Nike + Human Race at Soldier Field in Chicago. “If my friends have anything to say about it, it will probably be a Sox fan. So, knowing me, it will probably come out a mathematician.”
The Fall Out Boy bassist has definitely been studious in his fact-finding mission about fatherhood. Besides reading books including The Expectant Father, Wentz, 29, said one famous friend in particular has been a wealth of information.
“Joel [Madden] gives me advice,” Wentz said of the Good Charlotte rocker, who’s dad to 7-month-old daughter Harlow with girlfriend Nicole Richie. “[It’s] been really awesome for me to go to another person my age.”
During Fall Out Boy’s performance at the Nike + Human Race, Simpson – wearing a black blouse and purple pants – hung out backstage with friends. While the band played their cover of Michael Jackson’s “Beat It,” she started dancing with her arms around her belly.
So, could the song end up on Simpson’s labor playlist? “That’s probably going to be all up to Ashlee – what she picks,” Wentz said. “I couldn’t be like, ‘Oh, you’re going through labor, here’s the music I want you to listen to.’ So, it’s whatever she picks.”
That may include his own vocals. Wentz said he sings to the baby “late at night …. Baby’s gotten a couple lullabies.”
In a recent posting on Simpson’s blog, she joked that while she’s pregnant, her “closet full of size 0’s are being worn by Pete right now and he looks hot in them.”