The similarity heuristic is a lesser-known psychological heuristic pertaining to how people make judgments based on similarity.  More specifically, the similarity heuristic is used to account for how people make judgments based on the similarity between current situations and other situations or prototypes of those situations.

At its most basic level, the similarity heuristic is an adaptive strategy.  The goal of the similarity heuristic is maximizing productivity through favorable experience while not repeating unfavorable experiences.  Decisions based on how favorable or unfavorable the present seems are based on how similar the past was to the current situation.

For example, a person may use the similarity heuristic when deciding on a book purchase.  If a novel has a plot similar to that of novels read and enjoyed or the author has a writing style similar to that of favored authors, the purchasing decision will be positively influenced.  A book with similar characteristics to previously pleasurable books is likely to also be enjoyed, causing the person to decide to obtain it.

Problem solving in general is benefited by the similarity heuristic.  When new problems arise similar to previous problems, the similarity heuristic selects an approach that previously yielded favorable results.  Even if the current problem is novel, any similarity to previous issues will help choose a proper course of action.

Problem solving in Kryptos is ultimately not benefited by the similarity heuristic.  The duplication of cipher methods for K1 and K2 is likely an deviation from the plan made by James Sanborn likely with the intent of providing the word “abscissa”.  Without the creators’ statements (2005 WIRED Interview) that K4 was a different ciphering method than K1-3 it would be very likely for future solvers to attempt to re-use ciphering elements from the Morse code, K1, K2 or K3 in their solution attempts on K4.  Even still, there are many who are still attempting vigeneres and rotational transpositions as part of their methods.  Now we know why they are predisposed to do so.