Handedness mismatch between parents and kids

My recent paper in the Journal of Economic Perspectives on handedness emphasizes the fact that lefties appear to have cognitive and earnings deficits relative to righties. I also point out that the cognitive gaps appear only in the left-handed kids of right-handed mothers. Lefty kids of lefty moms seem quite similar to right-handed kids of right-handed moms.

I'm grateful that Peter Orszag, in his recent BloombergView column, highlighted a finding I referenced near the end of the paper, namely that right-handed kids of left-handed mothers also appear to have such cognitive deficits. In other words, mismatch between the handedness of parents and kids may be as important as handedness itself. This was a fact I noticed only late in the publication process, so I didn't have time in the paper to think more about it.

Any theories out there as to why kids whose handedness doesn't match their mothers' handedness should score 0.1 standard deviations lower in both math and reading than kids whose handedness matches their mothers'?  Is this picking up something about genes? In utero shocks? Increased difficulties of imitating writing or other physical tasks?