Blue sky optimistic or cautious & grounded : Nature or Nurture stronger ?

We are all born leaning to either being (a) basically optimistic about the new and the unknown or (b) leaning to being cautious and skeptical towards them : that's Nature talking - our genetic and epigenetic mix. But can our upbringing (and later our own adult life circumstances) restrain or encourage those innate tendencies --- ie does Nuture also play a big part ?