This chart from John Hawks shows how brain volume (y-axis) has increased gradually during the past 2 million years of human evolution. rs31480 is an interesting SNP because individuals homozygous for the ancestral “C” allele have slightly smaller cranial volumes than TT individuals (well, at least the individuals in this particular study do). The SNP is located in the IL3 gene which regulates proliferation in a variety of cell types including neural progenitor cells – thus possibly influencing the development of overall brain size.
Personal sidenote – I have a kind of a big head – literally – but 23andMe does not cover rs31480. So, yeah, that whole mystery remains.