Among numerical formats available to express probability, ratios (i.e., frequencies) are extensively employed in risk communication, due perhaps to an intuitive sense of their clarity and simplicity. The present thesis was designed to investigate how the use of superficially different but mathematically equivalent ratio formats affects the magnitude perception of the probability that is conveyed. In particular, focus of research was the influence that those expressions, when employed in risk communication of prenatal screening test results, have on prospective parents’ perceptions of the chance of having a Down syndrome-affected child. No clear evidence was found in the literature, on whether the choice of one of the equivalent ratio format that can be used to state a given probability does matter in terms of subjective perception of the chance. Indeed, existent studies deliver contrasting results, and theories elaborated on those basis point in diverging directions. These could be summarised in the suggestion, on the one hand, that people tend to neglect denominators in ratios (hence they judge 10 in 100 as larger than 1 in 10: “Ratio-bias” or “denominator neglect”) and, on the other hand, in a claim that people neglect numerators, rather than denominators (hence they rate 1 in 10 as larger than 10 in 100: “group-diffusion” or “reference group” effect). Nevertheless, implications of either group of theories could not entirely be transferred to the specific issue at study, mainly because of problems of ecological validity (type of scenario and stimuli, experimental design). Hence, provided the necessary adjustments to both the original experimental designs and materials, we tested empirically the applicability of those predictions to the specific case under examination. Subjective evaluations of equivalent ratios presented between-subjects in scenario paradigm were analysed by means of the magnitude assessments given by a total number of 1673 participants on Likert scales. Overall, results of a series of 12 main studies pointed to a new bias which we dubbed the “1 in X effect” given the triangulation of its source to that specific ratio format. Indeed, findings indicated, that laypeople’ subjective estimation of the same probability presented through a “1 in X” format (e.g., 1 in 200) and an “N in X*N” format (e.g., 5 in 1000) varied significantly and in a consistent way. In particular, a given probability was systematically perceived as bigger and more alarming when expressed in the first rather than in second format, an effect clearly inconsistent with the idea of denominator neglect. This effect was replicated across different populations and probability magnitudes. Practical implications of these findings for health communication have been addressed in a dedicated section, all the more necessary considering that in one study on health-care professionals we had found, that they appeared themselves de-sensitized to the “1 in X effect” (seemingly because of their daily use of probabilistic ratios). While the effect was not attenuated in laypeople by a classic communicative intervention (i.e., a verbal analogy), it disappeared with one of the most employed visual aids, namely an icon array. Furthermore, in a first attempt to pinpoint the cognitive processes responsible for the bias, the affective account stemming from literature on dual-process theories has not received support, contrary to our expectations. Hence, the most likely origin for the bias seems to reside either, as suggested by some inspections, in a specific motivation to process the information, and/or in the increased ability to see oneself or others as that affected when a “1 in X” format is processed. Clearly, further empirical research is needed in order to attain this cognitive level of explanation.