Occlusion-based monaural hearing dramatically reduces binaural loudness summation.

Evidence

In binaural loudness summation, diotic presentation of a sound usually produces greater loudness than monaural presentation. However, experiments using loudspeaker presentation with and without earplugs find that magnitude estimates of loudness are little altered by the earplug, suggesting a form of loudness constancy. — Dijk, Pim van, Physiology, Psychoacoustics and…, 1522

Categorization

• QUESTION: Is the cognitive element real, or is it confounded by residual low-frequency energy leaking past the occluded ear? The authors themselves flag both as explanations.
• corroborated: 2 independent passages support this claim — Dijk, Pim van, Physiology, Psychoacoustics and…, 1522