We report on the glass-forming abilities of the homologous series 1,2-diphenylcyclo-butene, pentene, hexene and heptene a series that retains the cis-phenyl configuration characteristic of the well glass former, o-terphenyl. We find that the glass forming ability shows a sharp maximum for the, six-membered ring and demonstrate that this trend in glass forming ability is a consequence of a maximum, for the 1, 2-diphenylcyclohexene, of the reduced glass transition temperature T-g/T-m. Since the nonmonotonic trend in T-g/T-m is entirely due to variations in T-m, we conclude that the design target for maximizing the glass forming ability across an homologous series should focus on the crystal stability and the factors that determine it.