If you take out prejudice, with "pure reason", it will get very hard for norms to develop in society
There will be too much unpredictability in people's behavior. Too heavy a reliance on law and order. A more fragile society
"Avoid relationships outside marriage"
"Don't have kids before marriage"
These prejudices can go such a long way in ensuring better public health, lower crime, greater familial stability
E.g. The bride goes to the husband's place after marriage
(at least in a ritual sense)
These rituals reduce our choices, and can greatly simplify decision making in the early months of marriage
But what the ritual does is -
It takes out options from the table...and lends a certain regularity to early marital life.
By removing choices, it enables couples to focus on other things
The surname of the kid is that of the father, no matter how distinguished the mother is
Again the convention reduces costs. Reduces endless debates / fights in every house. By offering a fait-accompli, it regularizes domestic life
An ancient English prejudice that aided the rise of the English nation in the late middle ages
By letting the eldest son inherit the entire estate, it greatly reduced transaction costs
Also clarifying things to each kid right at their birth
But our minuscule powers of reason don't see the implications fully
Primogeniture actually promoted social mobility and encouraged adventure, by letting younger kids to move out and chart their own course
What universal education and promotion of "reason" has done is -
We now have a society full of middle brow people who frown on ancient prejudices
But not smart enough to see through the deeper implications and the wisdom buried in them