People tend to be less cooperative if they perceive that the frequency of helpers in the population is lower.
pˈiːpəl tˈɛnd təbi lˈɛs koʊˈɑːpɚɹətˌɪv ɪf ðeɪ pɚsˈiːv ðætðə fɹˈiːkwənsi ʌv hˈɛlpɚz ɪnðə pˌɑːpjʊlˈeɪʃən ɪz lˈoʊɚ.