Elderly suicide rates: the importance of a non-linear relationship with distal risk and protective factors

The elderly population size is increasing worldwide due to prolonged life expectancy and falling birth rates. Traditionally, suicide rates increase with age. For example, a recent cross-national study of 62 developing and developed countries reported an increase in suicide rates with aging in males and females in 25 and 27 countries respectively (Shah, 2007a). Thus, suicides in the elderly are an important public health concern. While much is known about proximal (individual level) risk and protective factors for elderly suicides (e.g. Conwell et al., 1991; Cattell and Jolley, 1995; Harwood et al., 2001), less is known about more distal (societal or population level) risk and protective factors (Rehkopf and Buka, 2006). Moreover, detailed knowledge of these distal factors may have greater public health relevance for the development of comprehensive prevention strategies (Knox et al., 2004). Proximal risk and protective factors have generally been examined either descriptively in case-series of elderly suicide victims (Shah and De, 1998) or analytically in case-control studies (Waern et al., 2003). Such studies have primarily examined linear relationships between suicide and proximal risk and protective factors. This approach is understandable because: (1) in case-series studies the most prevalent characteristics of suicide victims have been interpreted to be the risk factors and the least prevalent protective factors; and (2) in case-control studies the statistically significant differences in the characteristics of suicide victims and case-controls have been interpreted to be the risk or protective factors. The vast majority of early studies of distal risk and protective factors has also taken a similar approach and mainly focused on identifying presence or absence of linear relationships between these factors and suicide rates. For example, in countries with high socio-economic status, including Japan, England, and Wales, lower societal socio-economic status was reported to be linearly associated with increased elderly suicide rates (Kennedy et al., 1999; Whitley et al., 1999; Gunnell et al., 2003; Fukuda et al., 2005), and there was an absence of such a linear relationship with one measure of socio-economic status in a recent study (Shah et al., 2008a). Presence or absence of such linear relationships may be genuine. However, it is also possible that: (1) some such linear relationships may be erroneous and they may mask genuine non-linear relationships; and (2) the absence of a linear relationship may lead to erroneous conclusion of an absence of a relationship, although this too may mask a genuine non-linear relationship. These issues are examined below because of potentially important implications for the development of preventive strategies at a societal level. There are certain circumstances when consideration should be given to examining non-linear relationships between distal risk and protective factors and elderly suicide rates. These include: