born. DS1 was picky from day one (even refused frozen BM and formula - only fresh stuff for him). DD drank formula happily, eats most stuff (at 14mths) - this might not last. But DH is also very picky, won't eat scrambled eggs, fish, pork, chicken or mushrooms, so I think the picky gene is genetic - just a more sensitive palate and a dislike of new tastes. Those who think it is made have just never had a picky eater!
However, there is an environmental effect - after repeated exposure most people will end up accepting food, which explains why kids who are repeatedly given, say, bug larvae, will accept it happily.
Toddler fear of new foods is also well-documented as a definite phase.
My theory is, your kid eats fishfingers but not salmon so you offer fishfingers again, and reinforce love of fishfingers. But if you never offered the fishfingers and only ever offered salmon (and nothing else) then they would eventually eat it. But in Western diets we have lots of choice, kids gravitate to the things they naturally like the most (and most kids seem to like bread, breaded food, dry-ish food as opposed to wet food, salty things (think crisps, cheese, pesto and even olives!) and sweet food) - we just reinforce it.