Gerald refers to a common and commonly overlooked phonological process that contributes to both the spelling and the pronunciation problem of possessives with nouns ending in –s or –z.  The process is called haplology and is the simplification of two identical or very similar sequential syllables to one.  For example, linguists sometimes call the process “haplogy,” a slightly lame pun that illustrates the process.  (There’s a whole set of such linguistic puns but that’s a different source of pain.)

We don’t like to say “Jesus’s robe” or “the Andrewses’ house” because of the sequence of sibilant syllables, and so these are frequently said as “Jesus’ robe” and “the Andrews’ house.”  I don’t think we can say with any certainty that the problem today is simply phonological or simply orthographic; those two domains tend to interact in complex, messy ways, and so judgments of careful writers and speakers will be similarly complex and messy.

Herb