Complex sentence: Harry Harlow set up an experiment with rhesus monkeys, presocial primates, in 1958; the results from this study showed that social encounters are necessary in order for the young monkeys to develop both mentally and sexually.
Split into simpler sentences: Harry Harlow set up an experiment with rhesus monkeys, presocial primates, in 1958. Results from this study showed that social encounters are necessary in order for the young monkeys to develop both mentally and sexually.