What effect does under-processing have on the outcome of a perm?

Under-processing in a perm results in weak curls that may fall out quickly or do not form properly due to insufficient chemical processing. When the hair is not left in the perm solution for the appropriate amount of time, the bonds in the hair do not break and reform correctly, leading to limp and undefined curls. This inadequacy can stem from factors like hair condition, temperature, or incorrect application of the solution.

Consequently, clients may find that their curls do not hold, tend to revert back to their original shape, or lack the desired bounce and volume. This outcome emphasizes the importance of following the manufacturer's guidelines for time and application for achieving beautiful, long-lasting curls. The other options describe issues related to over-processing or improper curl formation but do not accurately reflect the specific consequences of under-processing.