Final answer:
Morton Hunt's research from the 1970s had limited generalizability because it included only individuals with listed phone numbers, leading to a non-representative sample which could not accurately reflect the entire population's behaviors or attitudes.
Step-by-step explanation:
One reason that Morton Hunt's research results from the 1970s were not considered generalizable is because he only included people with listed phone numbers. This presents a source of bias as it does not account for individuals without listed phone numbers, including those with unlisted landlines and, more contemporaneously, the proliferation of cell phone users.
Because this selection process does not represent everyone within a population, the resulting sample is likely to lack diversity and may not reflect the views or experiences of groups not included, such as people without a phone or those who choose not to list their phone number. This issue could severely limit the applicability of the findings to the general population.