"In this study, we compared the ability of undergraduate students and guppies to discriminate the same numerical ratios,
both within and beyond the small number range. In both students and fish the performance was
ratio-independent for the numbers 1–4, while it steadily increased with numerical distance when larger numbers were presented.
http://www.plosone.org/article/info:doi/10.1371/journal.pone.0031923
cp. Elizabeth Spelke: Large number discrimination in 6-month-old infants:
http://www.sciencedirect.com/science/article/pii/S0010027799000669
Anm. ed.iiQii.de: a major reason for limited view of ppt-items, (exponential) implications on the future, etc.
(in german: Warum wir nur bis 3 zählen können...)