the old expression, "if you lie down with dogs, you get up with fleas"?
Socrates is making the argument that if you corrupt
those who will inherit your society you are really only harming yourself.
Soiling your own nest so to speak! This utilitarian argument is all the more
powerful if you consider the relatively small size (by geography and by
population) of Socrates' "country." The equivalent
of our modern mid-sized cities, Athens' numbers
were measured by the thousands not by the millions. Thus, personally worsening
those with whom you will have to live out your golden years could indeed
cause harm to yourself.