Fall Out Boy's "Centuries" was the rare pop hit by a rock act. To what do you credit its crossover?
Fall Out Boy's openness. Like when I had to have the Suzanne Vega sample [of "Tom's Diner"], because I'm a '90s folk lesbian in my heart. People say being a rock band on pop radio isn't cool, but does anyone remember that The Beatles and The Rolling Stones are the biggest bands of all time -- and it's because they were on pop radio?
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Speaking of The Beatles, you and Julia have four top 40 hits together. Are you a permanent team, a la Lennon-McCartney?
Almost. There are times when we work separately -- if it's a scheduling conflict or not the right vibe for a certain artist -- but we spend 80 percent of our time together at this point. We met in just a random session, and she’s the best thing that ever happened to me.
"Sorry" has been a game-changer for Bieber. How did the song come about?
That was just me and Julia in a room doing what we do. The song is very special because it's challenging normal gender roles on radio -- where we're allowing one of the biggest, hottest male pop stars in the world to be vulnerable and ask for forgiveness.
Gwen Stefani's new album also draws on deep introspection, with songs inspired by her divorce. Was it an emotional process for you?
I definitely cried a couple of times, especially working on the title track, which we just called "Truth.” It was tears of joy, but it also broke my heart [to see] the amount of pain she went through. We wrote about getting to a place of fragile happiness and fear of hope. It was beautifully devastating.
Can you tell us about working with Britney Spears on her new album?
It's been such a cool experience. Her voice is just so special, yeah, and she's a very underrated writer. That girl's melodies!
Who's left on your wish list?
Stevie Nicks, Stevie Nicks and... Stevie Nicks! [Laughs] I would also really love to work with Kanye West, because I think he's one of the best producers of all time. And if I could write a hook for Jay Z, that would be really cool.
You've said recently that radio has gotten cool again. Having the perspective of someone who's seen the business for the last decade, how did think that happen?
I think it happened because, when a couple cool songs sneak through, it makes people more brave to do cool shit. Also, for me, the A&R executives that I work closely with -- specifically Wendy Goldstein [from Republic Records] and Aaron Bay-Schuck from Interscope -- they want the cool shit and they have that fire to, not only sell records, but to change shit and make awesome music. For songwriters, that makes our lives so much more exciting.
What do you think is the most pressing issue facing the music industry today?
Streaming. It is the way and we have to figure out how everyone gets paid. The larger streaming grows, the less money there will be to get the music out there, which to me is the sad part. I don't need to be filthy f---ing rich, I just want people to still be able to find new music. And if all the money's gone, then they're not gonna be able to find it.
A version of this story originally appeared in the April 16 issue of Billboard.