This research demonstrated that lower subjective SES increased delay discounting. This suggests an association between subjective SES and how people wait for larger future rewards. Moreover, to the best of my knowledge, this study offers the first evidence that the slope of subjective SES is steeper in smokers than in non-smokers, and that discounting behavior is apparent in smokers with lower SES. Although the association between smoking behavior and impulsive choice in delay discounting is consistent with previous findings (e.g., Bickel et al. 1999; Ohmura et al. 2005), this study suggests a need to assess the association with an individual’s social class, particularly one indicated by class and hierarchy.
Whereas subjective SES was associated with delay discounting, no effect of objective SES was found. The unexpected findings on objective SES might result from the fact that the present research adopted a categorical measure of yearly income with 8 points in increments of 2,000,000 yen and did not ask for the participants’ exact income. Because the distribution was indeed positively skewed (see Table 1), asking about monthly income and using a categorical measure with a smaller range of money in smaller increments would be more appropriate to detect a relationship between income and delay discounting. Future work testing for samples with a wide range of income is needed.
Past studies have suggested a strong correlation between subjective SES and health indicators such as chronic illness, hypertension, and exercise habits. In addition, a correlation is usually found even if objective SES is controlled for (e.g., Adler et al. 2008). Researchers have assumed that this relationship between SES and health suggests that individuals with lower subjective standing are more vulnerable and have greater responsiveness to stress, reflecting their lack of resources to handle it (e.g., Adler and Snibbe 2003). Moreover, given that social status is associated with serotonergic function fostering impulsive and aggressive behaviors (e.g., Edwards and Kravitz 1997), and that the association leads to poor health behaviors (e.g., Matthews et al. 2010), the current findings demonstrating the relationship between lower subjective SES and delay discounting might align broadly with the findings of past studies on subjective SES and health outcomes in that impulsivity underlies the relationships.
Impulsivity also underlies an association between smoking and delay discounting (Bickel et al. 1999; Ohmura et al. 2005). Nevertheless, the subjective SES and smoking status interaction on delay discounting behavior in this research suggests that smokers and non-smokers do not differ in the behavior when their subjective SES is high. This may result from a sense of control, which is higher in people with higher subjective SES (Kraus et al. 2009), moderates an impulsive choice. However, given that previous studies on the relationship between smoking and discounting have mainly focused on heavy smokers who consume no less than 20 cigarettes per a day (e.g., Bickel et al. 1999), that there is no difference in discounting behavior between non-smokers and mild smokers (Ohmura et al. 2005), and that in the current study only 40 % (17 out of 42) of the smokers are defined as heavy ones, the effect of impulsivity induced by daily nicotine exposure might be weak so that the difference between smokers and non-smokers becomes negligible particularly in people with higher subjective SES.
Although the present research just focused on the effects of demographic factors (SES and smoking status) in delay discounting based on correlations among these factors, it is crucial to explore the underlying mechanisms in future work. The key factors would be impulsivity and sense of control. Future work should examine the role of these factors in the relationships among SES, smoking, and delay discounting and clarify the causal relationships.
There are some shortcomings to the present research. First, it was based on a hypothetical scenario. Although previous studies found no difference between real and hypothetical rewards in terms of delay discounting (e.g., Johnson and Bickel 2002), the effects of SES and smoking might be different if the individuals have to make a choice about real monetary rewards. Second, because this study did not manipulate the participants’ assessment of their relative socioeconomic rank in addition to the cross-sectional nature of the data, the possibility that several factors (e.g., the features of the community where the individuals live) that intervene in the perception of subjective SES may have produced an association with delay discounting cannot be denied. Further investigations that manipulate relative socioeconomic rank (e.g., Piff et al. 2010) will be needed to improve the current findings based on correlations. Finally, this study did not address the effects of SES and smoking on monetary losses, although typically, only monetary gains are examined in most studies. Discount rate for losses tends to be lower than discount rate for gains. Does the difference for outcome effect influence how SES and smoking status interact to delay discounting? In future research, it would be important to see whether the current findings could be extended to the discounting of monetary losses.