Yellowing baits

What causes white soft body swimbaits like a magdraft to start turning yellow? Heat, age, etc. ?

Yellowing of white plastics - It's long been suggested that ultraviolet (UV) light — the same light that gives us sunburns — initiates reactions in the backbone of the polymer's structure that cause the yellow color change.

They are made from Plastisol. Heat, moisture and uv light will cause white, clear/translucent baits to yellow over time. Different plastisol's with higher resin content (higher durometer) to make them harder or more firm will yellow quicker than baits with less resin in them.