In the book of _The First Three Minutes_ by Weinberg, at page 106-107, it is stated that 

> SECOND FRAME. The temperature of the universe is 30,000 million
> degrees Kelvin [...] The nuclear particle balance has conse- quently
> shifted to 38 per cent neutrons and 62 per cent protons.

> [...]

> THIRD FRAME. The temperature of the universe is 10,000 million degrees
> Kelvin. [...] The decreasing temperature has now allowed the
> proton-neutron balance to shift to 24 per cent neutrons and 76 per
> cent protons.

What is the reason for this balance shift between neutrons and protons exaclty ? and what does determine this rate of the change this shift ?