I guess the word gender needs to be defined, but that is part of the problem I suppose, since it can mean many different things to many different people. By gender, I do not mean anatomical physical sexual structures( like breasts, or a penis). Maybe the best definition I could conceive is: whether one identifies as man or women or w/e gender intermediate. The question is in the title, and I am aware that it could be due to both effects, and was curious about this question due to the existence of transgender people, which in my mind means gender is more of a social concept. But then some transpeople claim they have felt that they were a women or man at birth, which implies a biological origin. And even though both are story-based evidence, I was hoping for something more conclusive from this stackexchange. 
Is gender biologically or socially determined?
gender
Gender identity, or the gender that a person feels they are (regardless of physical makeup) is usually imprinted in the neural circuits in early childhood. This could be genetic, environmental, or a combination of both. Gender identity can also change at various points in life, and can change from moment to moment within the course of a day.There are no nice, neat holes to fit things into (no pun intended.) There are as many possibilities and configurations as there are people.As an example, the following link discusses these concepts in great detail and draws from multiple studies. References are provided on the 4th page....one's gender is on a continuum, a blending, analogous to a gray scale. But, our distribution of gender is bimodal, that is, most people are lumped at the two ends with only a minority in the middle. The great majority will see themselves as either male or female with all that implies. Probably more upsetting to our conventional view of gender than this fuzziness of gender roles is that we can be a MIX of male and female identities within the same individual. http://www.healthyplace.com/gender/inside-intersexuality/multi-dimensionality-of-gender/