Anyone knows why girls and boys are equal?

Are you sure believe is the right word? I think this has to do more with trust (http://www.differencebetween.net/language/words-language/difference-between-trust-and-believe/) of a institutional (https://www.igi-global.com/dictionary/institutional-trust/14813) and/or social (http://essedunet.nsd.uib.no/cms/topics/2/1/) type.

Beleive and trust are basically the same and interchangeable. They both means you act on something that you don't (at least) fully understand.
In God we trust.
It doesn't make difference if trust is built on prior experience with someone but still it's believe