In this case, to 'reflect' on means to 'examine' something. The sentence is basically saying, 'these studies allow us to separate inherent traits from environmental ones.'

Twin studies have allowed us to be more reflective about what people are actually born with and what's cased by experience. Hi Yeji, Research studies performed on twins have given us the data and knowledge to THINK AND FORM OUR OPINIONS about what people are actually born with and what is learned through our experience. Hope this helps. Cheers, Lance