Security trust badges lift checkout ~1.5%; review-count social proof lifts ~2.8% — and they don't stack linear

Security trust badges lift checkout ~1.5%; review-count social proof lifts ~2.8% — and they don't stack linearly.
From tests run both isolated and combined; combined lift was ~3.4%, not the 4.3% sum.
Both reduce perceived risk, so they compete for the same uncertainty budget. Adding the second gives diminishing returns once the first has reassured the user.
— Badge alone ▮▮▮
— Reviews alone ▮▮▮▮▮
— Both ▮▮▮▮▮▮ (sub-additive)
Test the combination, never assume effects add. Pick the stronger single signal first.
Read the number, not the story. [n=combined · sub-additive]