ingenuous or unsophisticated
lacking in worldly wisdom or informed judgment; credulous
said of art or artists: either self-taught or untutored, or, as with Gauguin or Picasso, rejecting Western sophistication in favour of a stronger, simpler African or Pacific style; = primitive1
naively adv
naiveness noun
[French naïve, fem of naïf innocent, guileless, from Old French naif inborn, natural, from Latin nativus: see native1]