Commonly used in the random crits argument, this bias states a negative experience has the equivalent mental effect as 3-6 positive ones (depending on your source). This can be expressed as a negative experience equals -3 and a positive experience = 1 as you would need 3 positive experiences to cancel out the negative experience (or change the negative number depending on your source, we want to give benefit of the doubt here by using the low value).

You can divide mvm players into 3 categories: No pyro, play pyro, yes pyro. The no pyro players do not want a pyro in their game and thus receive a negative experience should one exist (-3). A person playing pyro can be assumed to be having a good time and thus a positive experience (+1). For yes pyro it will depend on the person if it is a positive experience or not, however it will not be negative (0 or +1).

We can now make a table of teams based on this.

Assuming yes pyros have no positivity

Assuming yes pyros have +1 positivity

These tables do not list every possible team comp, however it will be most team comps as you are unlikely to see more than 2 pyros for most waves.

As seen by both tables there are more situations where the total negativity in a lobby outweighs the total positivity and thus the conclusion:

Don't play pyro outside a 4+ stack.