Here's an interesting paper about trust. In an fMRI study of a 2-player investor-trustee game, trust developed as the trustees learned of the benevolent gameplay of the investor. By the later stages of the games, trustees 'trusted' or anticipate the generous moves of their partners, by showing anticipatory activations in their caudate nuclei.
The head of the caudate is an interesting area because asymmetries have been noted in young ADHD subjects (the asymmetry becomes less notable heading into adolescence) and this is one area particularly active in studies of romantic love (below).
It's interesting to think that structural differences could contribute to differences in levels of trust. It's also a good reminder too, that others will trust you when you show a consistent benevolent or generous pattern of behavior toward them.
fMRI of Trust
Reward, Motivation and Romantic Love