The shield-holding idea is not well supported by the references.
Fisher's thesis "Psychosocal differences between left-handed and right-handed children" seems to mainly evaluating consequences not causation. She cites
- Coates 1996
- Hollingworth 1923
In the references section these are given as
- Coates, E.F. (1996). The left handed: "Their sinister" history.
- Hollingworth L.S. (1923). Special talents and defects, their significance for education.
In fact, the first paper was written by Costas, Elaine Fowler not by Coates. (I've put in a request for this paper)
Other theories about left-handedness claim that left-handers had an advantage in combat. It seems to me that these two ideas are both plausible and yet are contradictory. See Faurie and Raymond.
The location of the human heart is approximately central although it is skewed significantly to the left.
Images from 1 2 3
Any benefit from holding a shield in the left hand is therefore perhaps marginal.
The selection of handedness is at least partly (but far from wholly) genetic and its origins are not definitively known. No single idea is likely to wholly explain the phenomenon.