From a post by Bailywolf - from whose fingers inspiration drips like unto manna - on rpgnet, I hereby give the Bailey-Wolfe Psychological Scale. This scale is used to measure a species on seven axes. The further from human the species' rating is, the more difficult it would be to communicate with. Each axis is rated from -9 to +9.
The Bailey-Wolfe Scale
Neotropism: This is the tendancy of the species to accept new and unusual ideas, tropes, and people. It is rated from -9 (tradition-bound and xenophobic) to +9 ("Oooh! Shiny!")
Sociability: This is the tendancy of the species to form and maintain bonds with its own kind. It is rated from -9 (surly and individualist) to +9 (sheeplike)
Instinct: This is the tendancy of the species to rely on instinct as opposed to learning. It is rated from -9 (pure learning) to +9 (pure instinct)
Logic: This is the tendancy of the species to use logic as a tool. It is rated from -9 (extremely intuitive) to +9 (extremely analytical)
Foresight: This is the tendancy of the species to plan ahead. It is rated from -9 (very short term) to +9 (very long term)
Pattern Recognition: This is the tendancy of the species to act based on seeing a known pattern as opposed to thinking everything through. It is rated from -9 (must follow chain of thought) to +9 (acts instantly on perceiving any pattern)
Individuals vary. Personality Traits modify the species-wide tendancy by the amount given to the trait - if a species has a Sociablity rating of -3, and an individual of the species has the trait Helpful 3, the individual has a Sociability score of 0, which may be less than the Sociability score of a particularly surly member of a very sociable species.