No established liberal democracy has achieved sex balance in its national legislature. Scholars agree skewed candidate pools put forward by parties during elections cause sex-disproportionate seat distribution, but disagree as to whether disproportionality is caused by too few women aspirant candidates coming forward (supply) or party selectors preferring men (demand). This paper uses a multistage method to explore supply and demand during the British Labour party's candidate selection process. Rare data from three elections and 4622 aspirants allow for an unobstructed look inside the secret garden of politics and reveal the party is not fully feminized insofar that women aspirants are disproportionally filtered out of its selection process and are disproportionally underrepresented in its candidate pool. Testing reveals a lack of selector demand for women aspirants has a greater impact on women's underrepresentation than an undersupply of women aspirants, a finding which supports using sex quotas to level imbalanced candidate slates.