Those variants of synaesthesia that trigger colour are well studied, although comparatively less is known about variants that involve cognitive constructs such as personality types. Here we investigate sequence-personality synaesthesia (also known as ordinal linguistic personification, OLP) in which sequenced units (e.g., letters) become associated to personalities or genders. We present the first group study of this variant, showing similarities and differences between synaesthetes and non-synaesthetes. In Experiment 1, we show that synaesthetes differ from the general population in the phenomenology of their reports, the depth of their personality associations, and the consistency of those associations over time. In Experiment 2, we show that synaesthetes are similar to the general population in the underlying rules that link their personalities to letters. Specifically, we show that these mappings are not random, but are based on a shared rule system linking linguistic qualities of letters with quantitative dimensions of personality (based on Goldberg's Big Five personality traits; Goldberg, 1990, 1992). Synaesthetes tend to associate high-frequency letters with high agreeable and low neurotic personalities, and non-synaesthetes share these tendencies at an implicit level. Together, these data show that synaesthetes differ from the general population in phenomenological ways, but that their underlying mechanisms may be common to all people.