The three hypotheses:

Full cannibalization: Shorts views replace long-form time 1:1. Total YouTube time unchanged.
Pure expansion: Shorts generate new attention without taking from long-form.
Partial cannibalization + expansion: The realistic case. Some substitution, some new time.

How to test (same methodology as Meta Reels, with YouTube-specific wrinkles):

Holdout experiment. Some % of users don't get Shorts surfaced. Measure long-form time among holdout vs. treated.
Within-user decomposition. For each user, compare their long-form time pre- and post-Shorts adoption. Users with decreasing total time = cannibalizers; users with growing total time = expanders.
DiD on geographic rollout. Shorts launched at different times in different countries. Compare the long-form trajectory in early-rollout vs. late-rollout geos.

YouTube-specific considerations:

Monetization differences. Shorts CPM is ~1/5 of long-form CPM. So even if Shorts cannibalized fully on a time basis, revenue per minute is worse. That's the business question, not cannibalization per se.
Creator switching. Creators who post Shorts might post less long-form. Creator-level cannibalization is separate from user-level.
Discovery pathway. Shorts drive discovery of creators whose long-form users then watch. So Shorts may cause long-form watch time among other users even if it cannibalizes the Shorts-viewer's own long-form.
Competitive context. Shorts exists specifically because TikTok was eating YouTube's teen share. Without Shorts, the cannibalization happens to TikTok instead, even more painful.

What senior candidates add:

Revenue-weighted cannibalization. If Shorts has 1/5 the CPM but generates 3× the minutes, total revenue is 60% of baseline. That's a real loss unless it's offset by retention.
Retention-weighted cannibalization. Users who engage with both Shorts and long-form retain better than either alone. Maybe net-positive even with revenue decline.
Strategic framing. If killing Shorts means TikTok acquires the teen audience permanently, long-term cost far exceeds short-term cannibalization.
Reference YouTube's public statements on Shorts strategy (CEO letters, shareholder calls) — they've been upfront that cannibalization is real but acceptable.

What to tell leadership:

"Shorts cannibalizes long-form ~30% at the user level. Net revenue per user is down ~15% due to lower Shorts CPM. Retention of Shorts-engaged users is up 8%, which compounds over 12 months. Net-net, Shorts is probably 0-to-slightly-positive on revenue. The strategic case for Shorts is teen acquisition vs. TikTok, not direct revenue."

Is YouTube Shorts cannibalizing long-form videos?

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