When the gender ratio of early-career researchers is not equal to the gender ratio among established researchers, the null expectation for <i>α</i> is not necessarily zero.

Specifically, if most collaborations occur between career stages, there will be an excess of mixed-gender collaborations (α α > 0, red areas). However, the conditions required for strong gender homophily (i.e. the red areas) are quite restrictive, making it unlikely that this issue can fully explain the homophily observed in our study. Additionally, in research disciplines where between-career stage collaboration is common and there is a shortage of women among established researchers (i.e. the blue areas), our study will underestimate the strength of gender homophily. Contour lines mark increments of 0.1.