  In this piece, written for a general audience, we propose a mechanism for
quantum entanglement. The key ingredient is collider bias. In the language of
causal models, a collider is a variable causally influenced by two or more
other variables. Conditioning on a collider typically produces non-causal
correlations between its contributing causes. This phenomenon can produce
associations analogous to Bell correlations, in suitable post-selected
ensembles. Such collider artefacts may become real connections, resembling
causality, if a collider is 'constrained' (e.g., by a future boundary
condition). We consider the time-reversed analogues of these points in the
context of retrocausal models of QM. Retrocausality yields a collider at the
source of an EPR-Bell particle pair, and in this case constraint of the
collider is possible by normal methods of experimental preparation. It follows
that connections resembling causality may emerge across such colliders, from
one branch of the experiment to the other. Our hypothesis is that this
constrained retrocausal collider bias is the origin of entanglement. This piece
is based on a suggestion first made in arXiv:2101.05370v4 [quant-ph], and is an
ancestor of an essay now published online in Aeon magazine [Price & Wharton
2023a]. In an updated version of the argument in arXiv:2309.10969 [quant-ph] we
(i) demonstrate its application in a real Bell experiment; and (ii) show that
we can do without an explicit postulate of retrocausality
