Commonly, references to the number of sign language users in a given country are based on rough estimations. The intricacy of establishing reliable estimates is related to the circumstance that there is no one-to-one relationship between deafness and sign language use because not all individuals with a significant degree of hearing loss use sign language (Mitchell et al. 2006: 312; Johnston 2006).5 In addition, definitions of deafness and hearing impairment used in the statistics differ (for example, with respect to the hearing loss thresholds established for the distinct categories, or the conflation of two or more categories into one, as it might occur with the categories of “mild” and “moderate”, cf. Johnston 2006: 138) making it difficult to appropriately assess the development in a given country or to make comparisons of the estimates for different countries (Johnston 2006: 138). Hence, the commonly cited figure of 1 person per thousand (or 0,1 percen age) of severe to profound hearing impairment in developed countries remains a rough estimate. This holds equally of the much-cited figure of less than 10 % of deaf children being born to deaf parents (Mitchell & Karchmer 2006).
While deafness and sign language should not be conflated (Mitchell et al. 2006: 137), information on changes in the size of the deaf population can be used as an indicator for changes in the number of sign language users, which in turn, have an impact on the service provision targeting this population, such as education and interpretation.
In his detailed portrayal of the situation of the deaf community in Australia, Johnston identifies improved medical care, cochlear implants and genetic science among the factors that affect the decreasing incidence of deafness in that country. Analogous trends can be observed in other Western countries. Johnston (2006: 137) highlights the impact of these developments on sign bilingual education, training programmes for teachers of the deaf, and educational interpreting, 
apart from the consequences at the level of research, teaching and documentation of the language (Australian Sign Language in his case).
With respect to the impact of the developments in the medical area on the incidence of deafness, three issues are worthy of mention. Firstly, the decrease of the incidence of deafness in the 1980s is commonly related to the discovery of vaccines that allowed for the control of diseases causing hearing impairment, in particular, rubella. It should be noted that a higher incidence of deafness is commonly reported for the years 1964-1970, (Johnston 2006: 141), i.e. the years of the rubella epidemic. For Australia (2006: 141) Johnston acknowledges a tenfold decrease in the incidence of deafness from that period until today (i.e. 3.46 vs.
0.37 in the 1980s for all childhood deafness, although a higher rate is observed in the last years because of improved diagnostic testing). Secondly, the population of hearing impaired individuals has been affected by changes in the medical sciences leading to an increased survival rate of newborns with diseases or premature births, which is also reflected in an increase of deaf people with additional disabilities (Johnston 2006: 156). And thirdly, new estimates of the incidence of hearing impairment in early childhood have become available through another development in the medical area, namely, universal neonatal screening for hearing impairment. Johnston (2006: 144) summarises the main conclusions that can be drawn from the insights obtained through neonatal screening in Australia, the UK and the USA:
In sum, the prevalence of hearing impairment appears to be both greater and lesser than the earlier, commonly cited figure of 1 in 1,000 in developed societies. The rate is higher insofar as improved diagnostic testing is revealing many more cases of mild-to-moderate hearing loss that would have gone completely undetected in the past or at least would have remained undetected until much later in life. The category “mild" (hearing loss of25-40 dB) is capturing a population not described in the National Acoustic Laboratories study, and the Western Australian criteria include children with a hearing loss of 35 dB or greater. Significantly, the category of mild hearing impairment, according to Stredler-Brown (2003), accounts for 30 percent of the total number of cases. Universal neonatal screening is also revealing that hearing impairment continues to emerge in the later years of childhood.
In addition, Johnston (2006: 146) highlights the low incidence of profound deafness, about 10 % of all childhood deafness, although he also remarks that (a) this proportion needs to be considered in relation to an expanded hearing impaired population (including mild hearing impairment, due to improved diagnostics and documentation) and (b) that the incidence rate increases significantly with age (indicating that the neonatal screening does not capture the whole picture).
Taking up the issue of the number of sign language users, Johnston assumes that many severely deaf children, although educated in the mainstream, will learn sign language and relate to the deaf community later in their lives (Johnston 2006: 152). As for the majority of the 10 % of profoundly deaf children it is acknowledged that they will probably not profit from hearing aids and oral education, and become sign language users (Johnston 2006: 152).
Based on the information documenting the shrinking of the deaf community the question arises about the future of the community of sign language users. With respect to the situation in Australia Johnston (2006: 160) portrays a rather dim scenario. In the worst case scenario depicted by this author the deaf community is brought to a halt by a 75 rate of cochlear implantation and systematic implementation of genetic knowledge. According to this author, the impact of cochlear implantation is so strong that the decline of the community would also occur even if more children were bilingually educated.
On another level, the question arises about how the developments depicted will affect the development of the deaf communities qua linguistic minorities, if deafness and sign language gain a different significance, perhaps as one in many but not the primary identifier of deaf individuals. In a response article to Johnston (2006), Mitchell (2006: 217) wonders about how changes at the level of hearing aids (cochlear implantation), language(s) learned, educational placements, and social groups might affect the identity of deaf individuals, and ultimately the vitality of sign language and the community of its users:
... do deaf children and young adults develop a sense of self that is different from that of their hearing peers? If so, does this difference function primarily as a source of frustration when corrective or compensatory measures fail to function or become unavailable? Or is this difference between deaf youths and their hearing peers a fundamental experience that defines the self and the basis for identifying sympathetic others with whom to share a sense of community?
While the question has to remain unanswered for the time being, we turn our attention to the developments that have led to an increased public presence of the deaf community at the national and international levels, affecting also the perception of sign bilingualism at the socio-political level.
-  As Mitchell et al. (2006: 312) succinctly remark, "... American Sign Language is a social andlinguistic phenomenon, for which deafness is a necessary human condition motivating its sustained use (Johnston 2004), but an individual’s deafness is neither a necessary nor a sufficientcondition for becoming an ASL signer. Finding all those who use ASL at home requires a surveyof people without regard to their hearing status.”
-  In a response article to Johnston, Mitchell (2006: 215) expresses her doubts about the impactof screening results on decisions taken by the parents, as they "often help prospective parentsprepare for the birth of a child with special needs. I am not confident that genetic screening willeliminate childhood deafness, and it remains to be seen whether there will be a net reduction inthe incidence rate.”
-  According to Johnston (2006: 158) genetic screening and gene therapy have undergone an accelerated development in the last years. Genes relating to deafness have been identified, and,one of the objectives of research in this area would be the “correction” of these genes in fertilisedeggs, embryos and newborns.