Japanese deploys a singleton-geminate contrast in obstruents and nasals, but not in glides.Even though Japanese allows lexical nasal geminates, patterns of emphatic gemination show that Japanese avoids creating nasal geminates. Japanese therefore disfavors sonorant geminates in general, and glide geminates in particular. These phonological patterns of geminates are actually found in other languages as well, such as Ilokano (Hayes, 1989). This paper tests hypotheses about why speakers of these languages show these preferences. Concerning the distinction between obstruent geminates and sonorant geminates, Podesva (2002) hypothesizes that the phonological dispreference against sonorant geminates exists because these geminates are easily confused with corresponding singletons. This confusability problem arises because sonorants are spectrally continuous with flanking vowels, and consequently their constriction durations are difficult to perceive. Two non-speech perception experiments, Experiments I and II, confirm this hypothesis by showing that length distinctions of consonant intervals that are spectrally continuous with surrounding segments are difficult to perceive. Concerning the difference between nasal geminates and glide geminates, this paper builds on the finding by Kato et al. (1997) that given streams of sounds, listeners use amplitude changes to demarcate segmental boundaries. Experiments III and IV show that amplitude changes facilitate categorization and discrimination of short/long contrasts of consonantal intervals. These results are compatible with the fact that several languages disfavor glide geminates more than nasal geminates. Overall, the results of the four perception experiments reported here accord well with the cross-linguistic phonological patterning of geminates. We close this paper by discussing what the current results imply about how the phonetics-phonology works.