Acquired capability as a bridge from NSSI to suicide
Since the original conception of acquired capability for suicide [
32,
33], NSSI was highlighted as a particularly potent means of eroding an individual’s natural fear of pain, and thus their fear of attempting suicide. In accordance with our three-factor conceptualisation of this construct [
44], relationships have been observed, in the general population, between NSSI and reduced fear of death [
49], increased mental rehearsal of suicide plans [
39], and greater tolerance for pain [
65,
66]. As there is little research on acquired capability for suicide in autistic people, let alone its relationship with NSSI, our first analysis distilled the construct and examined its three facets as mediators of the relationship between the lifetime incidence of NSSI and lifetime suicide attempts. As per the aforementioned literature, our data corroborated relationships between higher incidence of NSSI and each facet of acquired capability. In similarity with the developers of the three-factor model [
44], our two groups showed indirect effects of NSSI on more numerous lifetime suicide attempts through the mediators of mental rehearsal and reduced fear of death. While the pathways between NSSI, acquired capability, and lifetime suicide attempts appeared to operate similarly between the two groups, two moderation effects of diagnosis revealed that autistic status was associated with a weaker relationship between lifetime NSSI and mental rehearsal, and a weaker indirect pathway from NSSI to suicide attempts via mental rehearsal. We suspect that both examples of weaker behavioural contingencies reflect greater mental rehearsal and reduced fear of death in the autistic group (two main effects),
irrespective of their engagement in NSSI. Our findings somewhat contradict those of one previous study [
52], which found no group differences between autistic and non-autistic people in relation to fear of death; however, they are consistent with the greater exposure of autistic people to physically painful and emotionally provocative events across their lifespans [
67‐
69].
Two findings stood in contrast to the traditional understanding of acquired capability [
33]. The first of these was the lack of association between greater pain tolerance and lifetime suicide attempts in both groups. It is highly possible that this reflects a disconnection between our two-item self-report measure and pain tolerance in naturalistic settings, and/or between pain reported in the present day and past suicide attempts. However, this finding actually corroborates broader scepticism around the relationship between pain tolerance and suicide and the inclusion of this facet within the acquired capability construct. Pain tolerance is the least specific and reliable differentiator of suicide attempts from suicide ideation [
56], with some studies even finding lower pain tolerance in individuals who had attempted suicide [
70,
71]. It is possible that these findings reflect real-world nuance which is difficult or impossible to capture experimentally, as might be the case if pain tolerance influenced an individual’s choice of approach to a suicide attempt [
71]. However, while the original conceptualisation of pain tolerance assumed this variable could only increase monotonically or stay stable, recent literature reflects that pain perception is influenced by numerous psychological variables [
72], that it fluctuates across NSSI episodes and over time in its relation to perceived capability for suicide [
70,
73]. The present study corroborates the unreliability of pain tolerance as a supposed prerequisite for suicide attempts.
A second finding contrary to the proposed role of acquired capability within the ideation-to-action trajectory [
33] was the presence of a
direct effect of lifetime NSSI on lifetime suicide attempts. While it contradicts theory, this finding is in fact again consistent with other empirical observations of direct associations between NSSI and suicide attempts in the general population [
40,
43,
74]. There are, at least, two potential interpretations of this direct effect. Firstly, it may reflect the need for a more multifaceted concept of “suicide capability” [
37,
75‐
77], which, fluctuating over time, could incorporate “baseline” capability (relatively static dispositional factors, such as genetically high pain threshold and low fear of death) as well as dynamic and situational factors (such as access to means [
56,
78,
79], recent exposure to suicide attempts in others [
75,
80]). Ribeiro and colleagues [
81] note that although in a broad and logical sense the desire and capability to attempt suicide
are necessary prerequisites for suicide attempts, our highly specific operationalisation of this desire and this capability do not, at present, sufficiently capture the complexity of these multitudinous variables and their interactions over time—hence why even in longitudinal investigations, suicide capability only explains a small degree of the variance in suicide attempts.
However, the second interpretation of this finding suggests we need to look further beyond acquired or broader suicide capability to understand alternative means through which NSSI might influence suicide risk. Firstly, NSSI is associated with thwarted belongingness and perceived burdensomeness [
38,
82,
83], both of which the Interpersonal Theory of Suicide asserts are necessary for suicide desire/ideation [
32,
33]. While NSSI is associated with poorer interpersonal problem-solving [
84] and interpersonal distress [
85], it has been suggested to exacerbate these states and may thus
contribute to suicide ideation [
38,
82,
83], which would be a necessary propellant for suicide capability to enable a suicide attempt. Similarly, although NSSI is associated with intolerance for psychological pain and negative affect [
86‐
88], some suggest it may exacerbate psychological pain by precluding the development of more effective coping strategies, such that individuals are vulnerable to suicide in a scenario where NSSI fails to provide adequate escape [
40,
43]. Of course, while NSSI may contribute to or exacerbate these states, an alternative interpretation lies in the possibility that these and other variables could operate as hidden factors in the relationship between NSSI and suicidality: “third” variables which, shared by both, might give rise to the appearance of a relationship between the two. For instance, in autistic populations, cognitive inflexibility [
89,
90] rumination [
91] and alexithymia [
22,
92] have been highlighted as potential factors in NSSI and suicidal behaviour (as well as broader psychopathology), perhaps in part because they preclude adaptive means of problem-solving and emotion regulation. In the general population, psychopathology has been suggested to wholly or partially mediate links between NSSI and thwarted belongingness, perceived burdensomeness and suicide ideation [
40,
82], and to explain unique variance in suicide attempts over and above contributions from thwarted belongingness, perceived burdensomeness, and acquired capability [
93,
94].
It is likely, given their differences in pain perception [
95‐
97] and lifetime experiences [
67‐
69], that the relevance and importance of suicide capability facets differ between autistic and non-autistic populations, just as the stability and rate at which capability develops may also differ. As both mental rehearsal and reduced fear of death partially mediated relationships with lifetime suicide attempts, it remains an important research target in autistic people. However, a comprehensive exploration of links between NSSI and suicide must incorporate additional factors beyond even the broader scope of suicide capability and must accommodate the element of indeterminacy, the reality that different combinations and interactions of factors can give rise to the same behavioural outcome [
81,
98].
Specific types and features of NSSI in relation to acquired capability and suicide attempts
On the assumption that an association does indeed exist between NSSI and later suicide risk, and that this connection is partially related to suicide capability, an important question is whether different NSSI behaviours differentially create suicide capability and whether they do so in a broad sense or only in relation to certain methods of suicide [
74]. Having previously observed a particular relationship between suicidality and cutting in autistic people [
31], we examined this and two other common NSSI behaviours in the light of features which might mark them as particularly worrisome in terms of acquired capability and future suicidality, most notably in relation to violence, painfulness and tissue damage [
33,
46,
49,
51]. Our mediation models examined the effect of each behaviour on lifetime suicide attempts as mediated by three sequential mediators: average pain experienced during NSSI, the extent to which individuals reported habituating to NSSI, and acquired capability as a whole construct. As we could not statistically compare autistic and non-autistic participants, differences between these sets of analysis may not be meaningful. We will therefore focus solely on autistic participants.
Interestingly, our binary endorsement of cutting and our continuous measure reflecting a range of NSSI behaviours behaved very similarly as variables: both were associated with habituation (albeit at trend level for cutting) and acquired capability; both directly predicted suicide attempts as well as exerting indirect effects via habituation and acquired capability sequentially, while cutting also exert an indirect effect via acquired capability alone. This is consistent with the previous literature examining these two predictors in non-autistic people. Among individual NSSI behaviours, cutting is particularly painful and provides immediate and lasting visual proof of physical damage, which is an important element of NSSI for some people [
99‐
101]. It also straightforwardly approximates one means of suicide which could be reached by an escalation of the same behaviour, such that it is a potent means of increasing pain tolerance and reducing fear of pain and death [
46,
102,
103]. Just as cutting has indeed been especially associated with acquired capability [
33,
104] and with suicide attempts in the general population [
49‐
51], so too is there a robust relationship between diversity of NSSI behaviours and suicide attempts [
8,
47,
105]. Theoretically, exposure to diverse methods would be predicted to result in an increased likelihood of habituation to a range of behaviours and types of pain [
46,
105]: while its association with self-reported pain tolerance is inconsistent [
40], NSSI range is indeed associated with reduced fear of death [
40,
43,
45,
48], and greater acquired capability as a whole [
104]. In our data, the existence of a
direct effect of both cutting and NSSI range on lifetime suicide attempts is an important indication of other variables which might explain these relationships. In non-autistic people who self-injure, both of these predictors have been linked to more severe psychopathology, greater emotion regulation difficulties, and poorer impulse control [
45‐
47,
104,
106]; the increased versatility associated with a range of behaviours may reflect increased need, willingness and ability to engage in NSSI even when preferred means are inaccessible [
46]. We cannot here determine the nature of this direct effect, but our findings are suggestive of extra risk of suicide associated with cutting and with engagement in diverse NSSI behaviours in autistic as in non-autistic people.
Interestingly, self-hitting influenced lifetime suicide attempts only via acquired capability, thus conforming most closely to original ideas around this construct [
33]. Self-hitting has received less experimental attention than other forms of NSSI, in part because it tends to occur as one of a repertoire rather than as a singular NSSI behaviour [
50,
104]. In non-autistic adolescents, Somer et al. [
107] did identify a group that primarily engaged in self-hitting. They likened this group to latent subgroups described as “mild” or “moderate NSSI” in other samples [
106,
108,
109], having a lower likelihood of psychological distress or psychopathology, lower likelihood of past suicide attempts and lower likelihood of other health risk behaviours (smoking and drinking) than groups characterised by skin-cutting or diverse means of NSSI. On the other hand, self-hitting may be more strongly associated with aggression, which has its own relationship with suicidality and acquired capability [
50]. This may explain why its effect on suicidality was solely mediated by this variable.
These analyses yielded several null findings and some unanswered questions other than the aforementioned direct effect. Severe scratching and/or pinching was unrelated to any other variables: it seems likely that this reflects the broad wording of this item on the NSSI-AT, which could conceivably apply to behaviours ranging in painfulness and destructiveness. Pain experienced during NSSI was not associated with any one predictor, mediator or with suicide attempts as an outcome. While this may reflect the aforementioned inconsistency around the role of pain tolerance in relation to acquired capability and suicide risk [
56], it is further notable that this index reflected pain experienced during NSSI in
general, rather than in relation to any one of these specific behaviours. The same is true of our measure of habituation to NSSI in general—but interestingly, our analyses of cutting and self-hitting both revealed two separate indirect effects of these predictors on lifetime suicide attempts, one via acquired capability alone and one via habituation and acquired capability sequentially (albeit much weaker than the former). Interpretation of this finding can only be speculative at present, but as habituation only affected suicide attempts via acquired capability rather than independently, it is possible that this effect reflects the overlap between the two constructs: the habituation subscale of the NSSI-AT might be expected to relay most closely to pain tolerance, but less to fear of death or mental rehearsal. While these analyses are generally supportive of a pathway from NSSI to suicide via acquired capability, they corroborate our first analysis in reflecting the existence of additional mechanisms through which NSSI, and particular forms of NSSI, might influence suicidal behaviours.
Limitations and future directions
The most prominent limitations of the present study relate to (a) the study design and operationalisation of variables, (b) the disparity between samples, and (c) their limited generalisability to wider populations. As pertains to the first of these points, our cross-sectional design was ill-equipped to test causal or directional hypotheses. While our analyses are suggestive of pathways between NSSI and lifetime suicide attempts via acquired capability, it is possible that capability was acquired
through suicide attempts as opposed to preceding them, given the increasing ease of subsequent attempts [
33,
48,
98]. Emerging views suggest that there are likely no
necessary prerequisites for suicide but instead many possible factors and combinations of factors, distal and proximate, and that these fluctuate over time [
81,
110]. Longitudinal or semi-longitudinal designs, or even computational modelling (e.g. [
98,
111,
112] may afford a clearer picture of multiple possible pathways from NSSI to suicide, including any causal contributions from acquired capability.
Relatedly, our operationalisation of key variables may have been inadequate. The interpersonal theory of suicide is suggestive of a “dose–response” relationship between NSSI and acquitted capability, though this has not received appropriate experimental scrutiny. Our analyses relating to specific NSSI behaviours were limited by the use of binary variables, indicative only of the presence or absence of the behaviour but not its frequency, extent or history; nor did these binary variables reflect how other variables, like the type and intensity of pain, the presence or absence of blood, and an individual’s psychological state during NSSI, might moderate a process of acquiring suicide capability [
8,
113]. Our operationalisation of pain typically experienced during NSSI, and the pain subscale of the ACWRSS, was likely inadequate. Indeed, there is presently no consensus around the optimal assessment of acquired capability in
any population [
56,
57]. The present study adopted a broader concept of acquired capability than previous investigations which focussed mainly or solely on the reduced feature of death [
52,
53], but all used measurement tools designed for non-autistic samples. Quite simply, the scope and nature of suicide capability is still unknown in autistic people, as is how it may interact with other autistic features. For instance, with better operationalisation of pain tolerance, it is possible that sensory sensitivities could differentially impact the contribution of this variable to suicidality in autistic people.
The strength of our conclusions is limited by several issues concerning sampling and recruitment. Firstly, we did not clinically validate self-reported autism diagnosis or
lack of autism diagnosis in the comparison group, relying on self-report only. Potentially greater issues lie in both the disparity between our samples and their generalisability to autistic and non-autistic populations. While we attempted to control for age and for sex in group comparisons, our groups occupied different lifespan stages: non-autistic participants as emerging adults, and autistic participants as approaching or navigating midlife. While little is known about changes in suicidal behaviour and NSSI across autistic lifespans, we know that the nature of NSSI and suicidal behaviour varies across the lives of non-autistic people [
114‐
117]. Our efforts to statistically control for age were indubitably inadequate as a means of counteracting the different life experiences and perspectives of the groups, weakening the validity of these comparisons.
With regards generalisability, our convenience sample of non-autistic undergraduates furthermore comprises a very specific cohort unrepresentative of the non-autistic population generally [
118]. Our autistic sample, too, is unrepresentative of many individuals within the autistic community. Those under- or unrepresented here include individuals with poor computer literacy; those with severe intellectual and/or communication impairments; individuals belonging to ethnic minority groups; and individuals with non-binary or transgender identities. Furthermore, a sampling bias may have been introduced if the study was more salient to those with a history of NSSI, suicide ideation or suicide attempts. Unusually for autism research, our sample was strongly skewed in favour of cisgender autistic women. The majority would be classified as “late-diagnosed”, with only 15 participants diagnosed at or below the age of 7 (a cut-off suggested in one recent study [
119]). As reflected in their qualifications, they likely corresponded to a profile with fair-to-strong camouflage and compensation abilities, more normative verbal style, and possibly stronger executive function than others within the autistic spectrum [
120‐
122]. Individuals with this profile seem disproportionately represented in studies which recruit via social media or other online methods [
123,
124]. As this is the approach adopted by the present study and several others in this relatively young field [
22,
30,
31,
52‐
54], findings are likely unrepresentative of all within the diverse autistic community. It is possible, given the differences noted between late- and early-diagnosed samples in mental health and wellbeing [
125,
126], that risk and protective factors for NSSI, suicidality and psychopathology differ as a function of age at diagnosis. The topography of NSSI and its relation to psychopathology and suicide risk remains an important target for future research, which could adopt more tailored recruitment strategies for underrepresented groups (e.g. [
127]).