A note on the parity conjecture and base change

Abstract: The parity conjecture predicts that the parity of the rank of an abelian variety is determined by its global root number, that is by the sign in the conjectural functional equation of its L-function. Assuming the Shafarevich-Tate conjecture, we show that if a semistable principally polarised abelian variety $A/\mathbb{Q}$ satisfies the parity conjecture over $\mathbb{Q}$ and over all quadratic fields, then it satisfies it over all number fields. More generally, we establish a criterion for when the parity conjecture for the base change of an abelian variety to a larger number field is already implied by the parity conjecture over the ground field and over other small extensions.