What is Psychological Safety?
Psychological safety is the shared belief that the team is safe for interpersonal risk — you can ask the naive question, admit the mistake, challenge the senior person, and report the bad news without being punished or embarrassed. Google's Project Aristotle found it the single strongest predictor of team effectiveness, ahead of talent and resources.
It is not comfort or niceness: safe teams argue hard about ideas precisely because nobody's standing is at stake. Leaders build it the only way it builds — by how they respond the next time someone brings a problem.
Worked example
A junior engineer notices the load calculation looks wrong the night before a pour. On a safe team, she says so at 9pm, the senior checks, finds a units error, and the pour waits a day — she's publicly thanked. On the unsafe team next door, the same engineer says nothing, remembering what happened to the last question-asker. Concrete cures either way; only one of those teams finds its errors while they're still cheap.