Chris Weller outlined an observation about social etiquette:

• In general people seem to follow a “rule of three” in group conversations. As long as at least three people are listening to a speaker speak, the rest of the group feels comfortable taking out their phone and focusing on that.
• The aim seems to be to make sure that the person talking feels as if they’re being listened to and isn’t being ignored.
• This then makes it more socially acceptable to break away from the conversation and focus on notifications from a phone.

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Source: Tech Insider