For similar reactions with an equal driving force, the difference in ∆E ‡ is simply the difference in ∆E ‡ int, leading to horizontal 'motion' of the product parabola, such that for an earlier TS a lower activation energy is expected (Figure ). In the context of small ring reactivity, we propose that electron delocalization within three-membered rings reduces this intrinsic activation barrier by increasing the polarizability of the ground state electron density, compared with four-membered ring analogues. Hait and Head-Gordon recently demonstrated that the polarizability of the electron density is maximized at or near a transition state due to electron delocalization accompanying partial bond cleavage and formation. Qualitatively, the relationship between this delocalization and reactivity can therefore be understood by considering how delocalization evolves during a bond breaking/making process: Attaining a delocalized electron arrangement at the TS will be facilitated if this bond is already partially delocalized in the ground state, resulting in an earlier, lower-energy TS.