Hyperconjugation was suggested as the reason for the increased stability of carbon-carbon double bonds as the degree of substitution increases.  Early studies in hyperconjugation were performed by in the research group of George Kistiakowsky. Their work, first published in 1937, was intended as a preliminary progress report of thermochemical studies of energy changes during addition reactions of various unsaturated and cyclic compounds.  The importance of hyperconjugation in accounting for this effect has received support from quantum chemical calculations.  The key interaction is believed to be the donation of electron density from the neighboring C–H σ bond into the π* antibonding orbital of the alkene (σ→π*).  The effect is almost an order of magnitude weaker than the case of alkyl substitution on carbocations (σ→p), since an unfilled p orbital is lower in energy, and, therefore, better energetically matched to a σ bond.  When this effect manifests in the formation of the more substituted product in thermodynamically controlled E1 reactions, it is known as Zaitsevs rule, although in many cases the kinetic product also follows this rule.  (See Hofmanns rule for cases where the kinetic product is the less substituted one.)
One set of experiments by Kistiakowsky involved collected heats of hydrogenation data during gas-phase reactions of a range of compounds that contained one alkene unit. When comparing a range of monoalkyl-substituted alkenes, they found any alkyl group noticeably increased the stability, but that the choice of different specific alkyl groups had little to no effect.
A portion of Kistiakowsky's work involved a comparison of other unsaturated compounds in the form of CH=CH(CH)n-CH=CH (n=0,1,2). These experiments revealed an important result; when n=0, there is an effect of conjugation to the molecule where the ΔH value is lowered by 3.5 kcal. This is likened to the addition of two alkyl groups into ethylene. Kistiakowsky also investigated open chain systems, where the largest value of heat liberated was found to be during the addition to a molecule in the 1,4-position.  Cyclic molecules proved to be the most problematic, as it was found that the strain of the molecule would have to be considered.  The strain of five-membered rings increased with a decrease degree of unsaturation. This was a surprising result that was further investigated in later work with cyclic acid anhydrides and lactones.  Cyclic molecules like benzene and its derivatives were also studied, as their behaviors were different from other unsaturated compounds.
Despite the thoroughness of Kistiakowsky's work, it was not complete and needed further evidence to back up his findings. His work was a crucial first step to the beginnings of the ideas of hyperconjugation and conjugation effects.