More nature than nurture IME. DS is obsessed with cars, trains and planes and not remotely interested in dolls. My MIL bought him 2 dolls to prepare him for dd's birth and he's never even looked at them.
Dd on the other hand, despite being surrounded by thousands of toy cars, never looks at them and only wants to cuddle her dolls.
Also I can see even at 14 months that dd is interested in nurturing other people, so when she's eating something she tries to feed bits to me. It would never occur to ds to do this - he just wolfs down his food and then wants to leave the table.
Obviously they are all individuals but I think there are big differences in boys' and girls' behaviour which can't be explained by conditioning.