Another cue is having the same family name, especially if rare, which has been found to increase helpful behavior.
ɐnˈʌðɚ kjˈuː ɪz hˌævɪŋ ðə sˈeɪm fˈæmɪli nˈeɪm, ɪspˈɛʃəli ɪf ɹˈɛɹ, wˌɪtʃ hˈæzbiːn fˈaʊnd tʊ ɪŋkɹˈiːs hˈɛlpfəl bᵻhˈeɪvjɚ.