The new engineer joins the standup. She watches how the others report their progress, quick and defensive, focused on covering themselves rather than surfacing problems. Within three weeks, she's doing the same thing. Nobody told her to. Nobody had to.
Teams are pattern machines. They absorb behavior and replicate it. One person starts checking email during meetings, and suddenly everyone's half-present. One person starts asking "what can I do to help?" and collaboration quietly improves. The patterns aren't written down anywhere. They live in the small moments. The raised eyebrow when someone suggests something unconventional. The way credit gets assigned. The speed at which bad news travels upward versus how long it hides. These patterns teach people how to behave far more effectively than any formal onboarding ever will.
This is both the problem and the opportunity. Problem: toxic patterns are incredibly persistent because they replicate themselves in new team members before anyone notices. Opportunity: healthy patterns work the same way. Change the pattern at one node and it can ripple outward. The question becomes: which patterns are you propagating, and are they the ones you want spreading?