Ordinal linguistic personification
Ordinal-linguistic personification (OLP, or personification for short) is a form of synesthesia in which ordered sequences, such as ordinal numbers, days, months and letters are associated with personalities (Simner & Hubbard 2006). Although this form of synesthesia was documented as early as the 1890s (Flournoy 1893; Calkins 1893) researchers have, until recently, paid little attention to this form (see History of synesthesia research).
Experiences and reports 
Mme L. reports that “1, 2, 3 are children without fixed personalities; they play together. 4 is a good peaceful woman, absorbed by down-to-earth occupations and who takes pleasure in them. 5 is a young man, ordinary and common in his tastes and appearance, but extravagant and self-centered. 6 is a young man of 16 or 17, very well brought up, polite, gentle, agreeable in appearance, and with upstanding tastes; average intelligence; orphan. 7 is a bad sort, although brought up well; spiritual, extravagant, gay, likeable; capable of very good actions on occasion; very generous. 8 is a very dignified lady, who acts appropriately, and who is linked with 7 and has much influence on him. She is the wife of 9. 9 is the husband of 8. He is self-centred, maniacal, selfish, thinks only about himself, is grumpy, endlessly reproaching his wife for one thing or another; telling her, for example, that he would have been better to have married a 9, since between them they would have made 18 – as opposed to only 17 with her… 10, and the other remaining numerals, have no personifications”. (Flournoy 1893, pp. 219–220)
Cakins (1893) describes a case for whom “T’s are generally crabbed, ungenerous creatures. U is a soulless sort of thing. 4 is honest, but… 3 I cannot trust… 9 is dark, a gentleman, tall and graceful, but politic under his suavity” (Calkins 1893, p. 454).
For synesthete MT “I [is] a bit of a worrier at times, although easy-going; J [is] male; appearing jocular, but with strength of character; K [is] female; quiet, responsible…” (Cytowic 2002, p. 298).
More recently AP has reported that February is “an introverted female”, while F is a “[male] dodgy geezer”. Similarly, May is reported to be “soft-spoken” and “girly” while M is an “old lady [who] natter[s] a lot”, and while August is “a boy among girls”, A is a female “mother type” (Simner & Holenstein 2007; Simner & Hubbard 2006).
Experimental studies 
Personifications tend to co-occur with grapheme-color synesthesia and share many of the characteristics that are definitional of synesthesia, such as being consistent over considerable time intervals and generating concurrents automatically (Simner & Holenstein 2007). To demonstrate that personifications are automatically evoked, Simner and Holenstein used a modified Stroop paradigm, in which a mismatch between the evoked personality and the gender leads to slower reaction times. To test this, synesthete AP was asked to report whether common names, like Brian or Betsy were male or female names. Because ‘b’ is a male letter for AP, she was faster to identify Brian as being a male name, and slower to identify Betsy as being a female name. These results demonstrate that personifications are automatically evoked, as are other forms of synesthesia. In a slightly different task, in which letters that evoke either male or female personifications are arranged into a stick figure of either a boy or a girl, reactions times are slower when the letters do not match the figure than when they do, again demonstrating the automaticity of this form of synesthesia (Sagiv et al. 2005).
Neural basis 
Very little is known about the neural basis of this form of synesthesia, but one possibility is that OLP arises from cross-talk in the region of the inferior parietal lobule between regions of the angular gyrus involved with representing ordinal sequences, and adjacent regions involved with the identification of personality and theory of mind near the supramarginal gyrus (Simner & Hubbard 2006). Alternatively, this form of synesthesia may arise from cross-talk in the region called the “retrosplenial cortex”.
Among the Pythagoreans 
The Pythagoreans, an ancient Greek cult of natural philosophers notable for their interest in numerology, personified the natural numbers by assigning gender to them; the odd numbers were male and the even female (Rucker 1988). However, given the prevalence of synesthesia (1 in 23, see the main synesthesia entry), and that this is consistent among the Pythagoreans, this is unlikely to be synesthesia, but rather a form of learned association.
In fiction 
In A Tree Grows in Brooklyn, Francie gives numbers personalities while learning basic math operations. She gives X and Y personalities as well.
- Calkins, M. W. (1893), "A Statistical Study of Pseudo-chromesthesia and of Mental-forms", American Journal of Psychology 5: 439–464
- Cytowic, R. E. (2002), Synesthesia: A Union of the Senses, 2nd ed, Cambridge, MA: MIT Press, ISBN 0-262-03296-1
- Flournoy, T. (1893), Des Phénomènes de Synopsie, Geneva and Paris: Alcan
- Sagiv, N.; Amin, M.; Olu-Lafe, O.; Ward, J. (2005), "Beyond Colour: Further Varieties of Synaesthetic Experience", 14th European Society for Cognitive Psychology meeting, Aug 31 - Sep 3, Leiden, The Netherlands
- Simner, J.; Holenstein, E. (2007), "Ordinal linguistic personification as a variant of synesthesia", Journal of Cognitive Neuroscience 19 (4): 694–703, doi:10.1162/jocn.2007.19.4.694, PMID 17381259
- Simner, J.; Hubbard, E. M. (2006), "Variants of synaesthesia interact in cognitive tasks: Evidence for implicit associations and late connectivity in cross-talk theories", Neuroscience 143 (3): 805–814, doi:10.1016/j.neuroscience.2006.08.018, PMID 16996695
- Rucker, R. (1995), Infinity and the Mind, Princeton University Press, ISBN 978-0-691-00172-2