I started out thinking that, but after considering it some more, I'm not sure it's just a halo effect.

Here's a chain of correlations: parents who leave their children alone at random (rather than in a structured and pre-planned way) are probably more impulsive. Trait-impulsivity is hereditary. Thus, leaving a (blood-related) child alone is predictive of that child taking more risks and being the kind of child who can't be left alone safely.

(On the other hand, leaving a child you're the adoptive parent or guardian of alone isn't predictive of anything.)