You launched Performance Max, ROAS looked great, and someone on the team called it a win. Then you noticed something: your branded search campaign's volume dropped the same week. That's not a coincidence. PMax has no brand exclusion controls by default, and its asset groups will happily bid on your own company name — traffic you'd get organically or for a few cents through a well-run branded search campaign anyway.
Google's algorithm doesn't distinguish between a conversion it created and one it intercepted. If someone Googles your brand name, ready to buy, and PMax's shopping or search inventory shows up first, PMax gets the credit. Your blended ROAS looks fantastic. Your actual incremental revenue hasn't moved at all.
Why This Happens (And Why Google Won't Fix It For You)
PMax is built to maximize conversion value across every inventory type — Search, Shopping, Display, YouTube, Discover, Gmail, Maps — using automated bidding and audience signals you feed it. It has no native brand exclusion setting the way standard Search campaigns do with negative keywords. There's a partial workaround (account-level negative keyword lists that can apply to PMax), but many advertisers never set it up, and even when they do, it doesn't fully stop brand terms from surfacing in Shopping or Display placements.
The incentive structure makes this worse. Automated bidding chases the cheapest conversions available to hit your target ROAS or CPA. Branded search clicks are almost always the cheapest, highest-intent conversions in your account. An algorithm optimizing for blended performance will gravitate toward them every time, because it's rewarded for volume and value, not for incrementality.
The Diagnostic: How to Find Out How Bad It Is
- Pull the Search Term Insights report inside the PMax campaign and filter for your brand name, product names, and close misspellings.
- Compare the percentage of PMax conversions tied to brand terms against your total PMax conversion count — this is your cannibalization estimate.
- Cross-reference with your branded Search campaign's impression share. A drop in branded Search impression share that lines up with PMax launch timing is a strong signal.
- Check Auction Insights on your branded Search campaign for overlap with Shopping/PMax placements on the same brand queries.
- Look at new-vs-returning customer segments in PMax conversion data if available — high existing-customer overlap on 'new customer' campaigns is another red flag.
Once you've quantified the overlap, don't just eyeball it — put a number on it. If 30% of PMax's reported conversions are branded, and you already run (or could run) a branded Search campaign at a $0.50 CPC, you're essentially paying PMax's blended rate for traffic that costs a tenth as much elsewhere.
Fixing the Budget Split
The fix isn't turning off PMax. It's forcing separation so each campaign gets credit only for what it actually generates.
- Build or maintain a dedicated branded Search campaign with exact/phrase match on your brand terms, tight budget, and near-100% impression share as the goal.
- Add an account-level negative keyword list covering your brand name and variants, applied to PMax where Google allows it.
- Set a maximum CPC ceiling or a separate 'new customer acquisition' goal on PMax to nudge its bidding away from cheap brand intercepts.
- Re-run the ROAS math using only non-branded conversions from PMax against its actual non-branded spend — this is your true incremental performance number.
- Report blended and segmented numbers separately internally, so no one mistakes brand cannibalization for genuine growth.
This isn't a one-time cleanup. PMax's targeting shifts as it re-learns from new conversion data, so brand leakage can creep back in after a few months of 'set and forget.' Recheck Search Term Insights on a recurring basis, not just at campaign launch.
The point of this exercise isn't to distrust automation categorically — it's to stop paying premium, blended-rate prices for conversions that were already walking through the door. Get that separation right and you'll finally see what PMax is actually contributing, instead of what it's taking credit for.