Positive, but not negative, treatment outcome expectancies mediate the relation between depressive symptoms and treatment-related pain reduction.

Previous research has shown that depression is associated with adverse recovery outcomes following work-related musculoskeletal injury. Treatment outcome expectancies have also been shown to predict recovery trajectories following musculoskeletal injury. The present study examined the role of positive and negative treatment outcome expectancies as mediators of the relation between depressive symptoms and treatment outcome for individuals receiving physical therapy for a musculoskeletal injury. The study sample consisted of 153 individuals who had sustained a work-related musculoskeletal injury to the back or neck within 6 months of enrolment. Participants completed self-report measures of depressive symptom severity, pain severity, and treatment outcome expectancies prior to treatment; pain severity was assessed again after 4 weeks of treatment. The results of this study were consistent with previous research showing significant relations between depressive symptom severity, pain severity and treatment outcome expectancies. Bootstrapping mediation analyses separately assessed the mediating roles of positive and negative treatment outcome expectancies on the relation between depressive symptoms and pain severity. Findings revealed that positive treatment outcome expectancies mediated the relation between depressive symptoms and pain severity, whereas negative treatment outcome expectancies did not. Discussion addresses potential pathways through which positive treatment outcome expectancies might influence pain outcomes. The findings suggest that intervention techniques aimed at increasing positive treatment outcome expectancies, rather than decreasing negative treatment outcome expectancies, might contribute to better recovery outcomes for individuals experiencing pain and depressive symptoms following a work-related musculoskeletal injury.

Risk-targeted behavioral activation for the management of work disability associated with comorbid pain and depression: a feasibility study.

PURPOSE: The purpose of the present study was to conduct a preliminary evaluation of the feasibility and impact of a risk-targeted behavioral activation intervention for work-disabled individuals with comorbid pain and depression. METHODS: The design of the study was a single-arm non-randomized trial. The sample consisted of 66 work-disabled individuals with comorbid pain and depression. The treatment program consisted of a 10-week standardized behavioral activation intervention supplemented by techniques to target two psychosocial risk factors for delayed recovery, namely, catastrophic thinking and perceptions of injustice. Measures of pain severity, depression, catastrophic thinking, perceived injustice, and self-reported disability were completed pre-, mid-, and post-treatment. Satisfaction with treatment was assessed at post-treatment. Return to work was assessed at 6-month follow-up. RESULTS: The drop-out rate was 18%. At treatment termination, 91% of participants indicated that they were "very" or "completely" satisfied with their involvement in the treatment program. Significant reductions in pain (Cohen's d = 0.71), depression (d = 0.86), catastrophic thinking (d = 1.1), and perceived injustice (d = 1.0) were observed through the course of treatment. In multivariate analyses, treatment-related reductions in depression, catastrophic thinking, and perceived injustice, but not pain, contributed significant unique variance to the prediction of return-to-work outcomes. CONCLUSIONS: Risk-targeted behavioral activation was found to be an acceptable and effective intervention for work-disabled individuals with comorbid pain and depression. The findings suggest that interventions targeting psychosocial risk factors for pain and depression might contribute to more positive recovery outcomes in work-disabled individuals with comorbid pain and depression. TRIAL REGISTRATION: ClinicalTrials.gov: NCT0517442 . Retrospectively registered.

The relation between perceived injustice and symptom severity in individuals with major depression: A cross-lagged panel study.

BACKGROUND: Perceived injustice has been associated with problematic recovery outcomes in individuals with debilitating health conditions. However, the relation between perceived injustice and recovery outcomes has not been previously examined in individuals with debilitating mental health conditions. The present study examined the relation between perceived injustice and symptom severity in individuals undergoing treatment for Major Depressive Disorder (MDD). METHODS: The study sample consisted of 253 work-disabled individuals with MDD who were referred to an occupational rehabilitation service. Participants completed measures of depressive symptom severity, perceived injustice, catastrophic thinking, pain and occupational disability at three time-points (pre-, mid- and post-treatment) during a 10-week behavioural activation intervention. RESULTS: Regression analysis on baseline data revealed that perceived injustice contributed significant variance to the prediction of depressive symptom severity, beyond the variance accounted for by time since diagnosis, pain severity and catastrophic thinking. Prospective analyses revealed that early treatment reductions in perceived injustice predicted late treatment reductions in depressive symptom severity. LIMITATIONS: The study sample consisted of work-disabled individuals with MDD who had been referred to an occupational rehabilitation service. This selection bias has implications for the generalizability of findings. CONCLUSION: The findings suggest that perceived injustice is a determinant of symptom severity in individuals with MDD. The inclusion of techniques designed to reduce perceived injustice might augment positive treatment outcomes for individuals receiving treatment for MDD.

The Relationship Between Level of Catastrophizing and Mental Health Comorbidity in Individuals With Whiplash Injuries.

OBJECTIVES: Pain catastrophizing has been shown to be correlated with measures of mental health problems such as depression and post-traumatic stress disorder (PTSD). However, the clinical implications of findings reported to date remain unclear. To date, no study has been conducted to determine meaningful cut-scores on measures of catastrophizing indicative of the heightened risk of mental health comorbidity. One objective of the present study was to identify the cut-score on the Pain Catastrophizing Scale (PCS) indicative of the heightened risk of the comorbidity of depression and PTSD. A second objective was to determine whether mental health comorbidity mediated the relationship between catastrophizing and occupational disability. MATERIALS AND METHODS: The sample consisted of 143 individuals with whiplash injuries. Pain severity, pain catastrophizing, depression, and post-traumatic stress symptoms were assessed after admission to a rehabilitation program. Mental health comorbidity was operationally defined as obtaining a score above the clinical threshold on measures of depressive and/or post-traumatic stress symptom severity. RESULTS: A receiver operating characteristic curve analysis revealed that a PCS score of 22 best distinguished between participants with and without mental health comorbidity. Results also revealed that mental health comorbidity mediated the relationship between catastrophizing and occupational disability. DISCUSSION: The findings suggest that a score of ≥22 on the PCS should alert clinicians to the possibility that patients might also be experiencing clinically significant symptoms of depression or PTSD. Greater attention to the detection and treatment of mental health conditions associated with whiplash injury might contribute to more positive recovery outcomes.

Clinimetric properties of the Nepali version of the Pain Catastrophizing Scale in individuals with chronic pain.

BACKGROUND: Pain catastrophizing is an exaggerated negative cognitive response related to pain. It is commonly assessed using the Pain Catastrophizing Scale (PCS). Translation and validation of the scale in a new language would facilitate cross-cultural comparisons of the role that pain catastrophizing plays in patient function. PURPOSE: The aim of this study was to translate and culturally adapt the PCS into Nepali (Nepali version of PCS [PCS-NP]) and evaluate its clinimetric properties. METHODS: We translated, cross-culturally adapted, and performed an exploratory factor analysis (EFA) of the PCS-NP in a sample of adults with chronic pain (N=143). We then confirmed the resulting factor model in a separate sample (N=272) and compared this model with 1-, 2-, and 3-factor models previously identified using confirmatory factor analyses (CFAs). We also computed internal consistencies, test-retest reliabilities, standard error of measurement (SEM), minimal detectable change (MDC), and limits of agreement with 95% confidence interval (LOA95%) of the PCS-NP scales. Concurrent validity with measures of depression, anxiety, and pain intensity was assessed by computing Pearson's correlation coefficients. RESULTS: The PCS-NP was comprehensible and culturally acceptable. We extracted a two-factor solution using EFA and confirmed this model using CFAs in the second sample. Adequate fit was also found for a one-factor model and different two- and three-factor models based on prior studies. The PCS-NP scores evidenced excellent reliability and temporal stability, and demonstrated validity via moderate-to-strong associations with measures of depression, anxiety, and pain intensity. The SEM and MDC for the PCS-NP total score were 2.52 and 7.86, respectively (range of PCS scores 0-52). LOA95% was between -15.17 and +16.02 for the total PCS-NP scores. CONCLUSION: The PCS-NP is a valid and reliable instrument to assess pain catastrophizing in Nepalese individuals with chronic pain.

Cross-sectional and Prospective Correlates of Recovery Expectancies in the Rehabilitation of Whiplash Injury.

OBJECTIVES: Investigations have shown that expectancies are significant prognostic indicators of recovery outcomes following whiplash injury. However, little is currently known about the determinants of recovery expectancies following whiplash injury. The purpose of the present study was to examine the cross-sectional and prospective correlates of recovery expectancies in individuals admitted to a rehabilitation program for whiplash injury. MATERIALS AND METHODS: Participants (N=96) completed measures of recovery expectancies, psychosocial variables, symptom severity, symptom duration, and disability at time 0 (admission) and time 1 (discharge). RESULTS: Consistent with previous research, more positive recovery expectancies at time 0 were related to reductions in pain at time 1 (r=-0.33, P<0.01). Scores on measures of pain catastrophizing, fear of movement and reinjury, and depression were significantly correlated with recovery expectancies. Pain severity, duration of work disability, and neck range of motion were not significantly correlated with recovery expectancies. Over the course of treatment, 40% of the sample showed moderate to large changes (an increase of ≥20%) in recovery expectancies, there were small changes (<20%) in 30% of the sample, and negative changes in 20% of the sample. A hierarchical regression showed that decreases in fear of movement and reinjury (ß=-0.25, P<0.05) and pain catastrophizing (ß=-0.23, P<0.05) were associated with increases in recovery expectancies through the course of treatment. CONCLUSIONS: The discussion addresses the processes linking pain-related psychosocial factors to recovery expectancies and makes recommendations for interventions that might be effective in increasing recovery expectancies.

Assessing catastrophic thinking associated with debilitating mental health conditions.

PURPOSE: The present study examined the psychometric properties of the Symptom Catastrophizing Scale (SCS). The SCS items were drawn from the Pain Catastrophizing Scale but were modified to make them better suited to the context of debilitating mental health conditions that are not necessarily associated with pain. The number of items was reduced from 13 to 7, and the response scale was simplified. METHODS: The SCS was administered to individuals diagnosed with Major Depressive Disorder (MDD) (N = 79) or with a chronic musculoskeletal (MSK) condition (N = 88). RESULTS: Exploratory factor analyzes revealed single factor solutions of the SCS for both the MSK and MDD samples. The internal consistency of the SCS was good. The SCS was significantly correlated with measures of pain severity, depressive symptom severity and disability in both samples. Individuals with MDD scored higher on the SCS than individuals with MSK. The SCS was shown to be sensitive to treatment-related reductions in catastrophic thinking. CONCLUSIONS: Preliminary analyzes suggest that the SCS is a reliable and valid measure of symptom-related catastrophic thinking associated with debilitating mental health conditions. Implications for Rehabilitation Although catastrophic thinking has been identified as a risk factor for disability, current assessment tools are not well suited for individuals with debilitating mental health conditions. This paper describes a brief assessment instrument that can be used to assess catastrophic thinking in individuals with debilitating mental health conditions. The results of this study suggest that targeting catastrophic thinking might yield reductions in symptom severity and disability in work-disabled individuals with major depressive disorder.

Return to work helps maintain treatment gains in the rehabilitation of whiplash injury.

This study examined the relation between return to work and the maintenance of treatment gains made over the course of a rehabilitation intervention. The study sample consisted of 110 individuals who had sustained whiplash injuries in rear collision motor vehicle accidents and were work-disabled at the time of enrolment in the study. Participants completed pre- and post-treatment measures of pain severity, disability, cervical range of motion, depression, posttraumatic stress symptoms, and catastrophizing. Pain severity was assessed again at 1-year follow-up. At 1-year follow-up, 73 participants had returned to work and 37 remained work-disabled. Analyses revealed that participants who returned to work were more likely to maintain treatment gains (77.5%) than participants who remained work-disabled (48%), χ = 6.3, P < 0.01. The results of a regression analysis revealed that the relation between return to work and the maintenance of treatment gains remained significant (ß = 0.30, P < 0.01), even when controlling for potential confounders such as pain severity, restricted range of motion, depression, and pain catastrophizing. The Discussion addresses the processes by which prolonged work-disability might contribute to the failure to maintain treatment gains. Important knowledge gaps still remain concerning the individual, workplace, and system variables that might play a role in whether or not the gains made in the rehabilitation of whiplash injury are maintained. Clinical implications of the findings are also addressed.

Sensitivity to Movement-Evoked Pain and Multi-Site Pain are Associated with Work-Disability Following Whiplash Injury: A Cross-Sectional Study.

Objectives Previous research has shown that sensitivity to movement-evoked pain is associated with higher scores on self-report measures of disability in individuals who have sustained whiplash injuries. However, it remains unclear whether sensitivity to movement-evoked pain is associated with work-disability. The aim of the present study was to examine the relation between sensitivity to movement-evoked pain and occupational status in individuals receiving treatment for whiplash injury. Methods A sample of 105 individuals with whiplash injuries participated in a testing session where different measures of pain (i.e. spontaneous pain, multi-site pain, sensitivity to movement-evoked pain) were collected during the performance of a simulated occupational lifting task. Results Hierarchical logistic regression analysis revealed that the measures of multisite pain and sensitivity to movement-evoked pain made significant independent contributions to the prediction of work-disability. Discussion The findings suggest that including measures of multisite pain and sensitivity to movement evoked pain in assessment protocols has the potential to increase the value of pain assessments for the prediction of occupational disability associated with whiplash injury. Clinical and theoretical implications of the findings are addressed.

The Relation Between Catastrophizing and Occupational Disability in Individuals with Major Depression: Concurrent and Prospective Associations.

Background Catastrophic thinking has been associated with occupational disability in individuals with debilitating pain conditions. The relation between catastrophic thinking and occupational disability has not been previously examined in individuals with debilitating mental health conditions. The present study examined the relation between catastrophic thinking and occupational disability in individuals with major depression. Methods The study sample consisted of 80 work-disabled individuals with major depressive disorder (MDD) who were referred to an occupational rehabilitation service. Participants completed measures of depressive symptom severity, catastrophic thinking and occupational disability at admission and termination of a rehabilitation intervention. Return-to-work outcomes were assessed 1 month following the termination of the rehabilitation intervention. Results Cross-sectional analyses of admission data revealed that catastrophic thinking contributed significant variance to the prediction of self-reported occupational disability, beyond the variance accounted for by depressive symptom severity. Prospective analyses revealed that reductions in catastrophic thinking predicted successful return to work following the rehabilitation intervention, beyond the variance accounted for by reductions in depressive symptom severity. Conclusions The findings suggest that catastrophic thinking is a determinant of occupational disability in individuals with major depressive disorder. The findings further suggest that interventions designed to reduce catastrophic thinking might promote occupational re-integration in individuals with debilitating mental health conditions.

Catastrophizing and pain-related fear predict failure to maintain treatment gains following participation in a pain rehabilitation program.

The present study explored whether pain-related psychosocial risk factors played a role in determining whether treatment gains were maintained following participation in a rehabilitation intervention for musculoskeletal injury. The study sample consisted of 310 individuals (163 women, 147 men) with work-related musculoskeletal conditions who were enrolled in a physical rehabilitation program. Measures of pain severity, pain catastrophizing and pain-related fear were completed at the time of admission and at the time of discharge. Pain severity was assessed again at 1-year postdischarge. Participants were classified as "recovered" if they showed a decrease in pain of at least 2 points and rated their pain at discharge as less than 4/10. Recovered participants were considered to have failed to maintain treatment gains if their pain ratings increased by at least 2 points from discharge assessment to 1-year follow-up, and they rated their pain as 4/10 or greater at 1-year follow-up. The results of a logistic regression revealed that participants with high posttreatment scores on measures of catastrophizing and fear of pain were at increased risk of failing to maintain treatment gains. The findings suggest that unless end-of-treatment scores on catastrophizing and fear of pain fall below the risk range, treatment-related reductions in pain severity may not be maintained in the long term. The clinical and theoretical implications of the findings are discussed.

Expectancies Mediate the Relations Among Pain Catastrophizing, Fear of Movement, and Return to Work Outcomes After Whiplash Injury.

UNLABELLED: Pain catastrophizing and fear of movement have been identified as key predictors of prolonged work disability after whiplash injury. However, little is known about the processes by which pain catastrophizing and fear of movement affect return to work. This study investigated the mediating role of expectancies on the relations between pain catastrophizing and return to work, and between fear of movement and return to work after whiplash injury. The study sample consisted of 154 individuals with whiplash injury who were enrolled in a multidisciplinary pain rehabilitation program. Participants completed measures of pain catastrophizing, fear of movement, and return-to-work expectancies after admission to a rehabilitation program. A follow-up telephone interview was used to assess work status 1 year after discharge. Consistent with previous research, analyses revealed that expectancies, pain catastrophizing, and fear of movement were significant predictors of return to work at 1-year follow-up. Regression analyses (bootstrapping) revealed that expectancies partially mediated the relation between catastrophizing and return to work. Expectancies completely mediated the relation between fear of movement and return to work. The significant predictive and mediating role of expectancies on return to work argues for the inclusion of expectancies as a specific target of intervention for individuals with whiplash injury. PERSPECTIVE: The findings suggest that expectancies might be part of the pathways by which pain catastrophizing and fear of movement affect return-to-work outcomes after whiplash injury. The findings argue for greater attention to return-to-work expectancies as a risk factor for problematic recovery outcomes as well as a target of intervention.

The Mediating Role of Recovery Expectancies on the Relation Between Depression and Return-to-Work.

PURPOSE: Depressive symptoms have been identified as a significant risk factor for prolonged disability, however, little is known about the process by which depression impacts recovery following work-related musculoskeletal disorders (WRMDs). The primary objective of this study was to examine whether recovery expectancies mediate the relation between depression and return-to-work (RTW) status in individuals with WRMDs. METHODS: A sample of 109 patients with WRMDs were recruited from 1 of 6 primary care physiotherapy clinics. Participants completed measures of pain severity, depression and recovery expectancies. RTW status was assessed by telephone interview 1 year after the initial assessment. RESULTS: Consistent with previous research, more severe depressive symptoms and lower recovery expectancies were associated with a lower probability of RTW. Logistic regression analyses revealed that recovery expectancies completely mediated the relation between depression and RTW status at 1-year follow-up. CONCLUSION: The results suggest that interventions specifically targeting recovery expectancies in individuals with WRMDs and depressive symptoms might improve RTW outcomes.

Catastrophizers with chronic pain display more pain behaviour when in a relationship with a low catastrophizing spouse.

The present study examined the relationship between couple concordance of catastrophizing and adverse pain outcomes. Possible mechanisms underlying the relationship between couple concordance of catastrophizing and pain outcomes were also explored. Fifty-eight couples were recruited for the study. The chronic pain patients were filmed while lifting a series of weighted canisters. The spouse was later invited to view the video and answer questions about the pain experience of their partner. Median splits on Pain Catastrophizing Scale scores were used to create four 'catastrophizing concordance' groups: low catastrophizing patient-low catastrophizing spouse; low catastrophizing patient-high catastrophizing spouse; high catastrophizing patient-low catastrophizing spouse; and high catastrophizing patient-high catastrophizing spouse. Analyses revealed that high catastrophizing pain patients who were in a relationship with a low catastrophizing spouse displayed more pain behaviours than patients in all other groups. These findings suggest that high catastrophizing chronic pain patients may need to increase the 'volume' of pain communication to compensate for low catastrophizing spouses' tendency to underestimate the severity of their pain experience. Patients' perceived solicitousness and punitive response from the spouse could not explain the group differences in pain behaviour. Theoretical and clinical implications of the findings are discussed.

Norming of the Tampa Scale for Kinesiophobia across pain diagnoses and various countries.

The present study aimed to develop norms for the Tampa Scale for Kinesiophobia (TSK), a frequently used measure of fear of movement/(re)injury. Norms were assessed for the TSK total score as well as for scores on the previously proposed TSK activity avoidance and TSK somatic focus scales. Data from Dutch, Canadian, and Swedish pain samples were used (N=3082). Norms were established using multiple regression to obtain more valid and reliable norms than can be obtained by subgroup analyses based on age or gender. In the Dutch samples (N=2236), pain diagnosis was predictive of all TSK scales. More specifically, chronic low back pain displayed the highest scores on the TSK scores followed by upper extremity disorder, fibromyalgia, and osteoarthritis. Gender was predictive of TSK somatic focus scores and age of TSK activity avoidance scores, with male patients having somewhat higher scores than female patients and older patients having higher scores compared with younger patients. In the Canadian (N=510) and Swedish (N=336) samples, gender was predictive of all TSK scales, with male patients having somewhat higher scores than female patients. These norm data may assist the clinician and researcher in the process of decision making and treatment evaluation.

Repetition-induced activity-related summation of pain in patients with fibromyalgia.

This study compared individuals with fibromyalgia (FM) and individuals with chronic low back pain (CLBP) on repetition-induced summation of activity-related pain (RISP). Fear of movement, pain catastrophizing and depression were examined as potential mediators of group differences. The sample consisted of 50 women with FM and 50 women with CLBP who were matched on age, pain severity and pain duration. Participants were asked to lift a series of 18 weighted canisters. In one trial, participants were asked to rate their pain after each lift. In a second trial, participants estimated the weight of each of the canisters. An index of repetition-induced summation of pain was derived as the change in pain ratings across repeated lifts. Analyses revealed that women with FM obtained higher scores on the index of RISP than women with CLBP. The heightened sensitivity to RISP in individuals with FM was not due to generalized hyperalgesia or a greater work output. Consistent with previous research, fear of movement was positively correlated with RISP. Pain disability was also associated with greater RISP, but not pain catastrophizing or depression. Discussion addresses the processes by which individuals with FM might have increased RISP responses. The findings of this study point to possible neurophysiological mechanisms that could help explain the high levels of pain-related disability seen in individuals with FM. Patients with fibromyalgia showed greater activity-related summation of pain than patients with chronic low back pain.

Judgments about pain intensity and pain genuineness: the role of pain behavior and judgmental heuristics.

UNLABELLED: The primary objective of the present study was to examine the relative importance of pain behaviors and judgmental heuristics (eg, gender stereotypes) in observers' inferences about pain intensity and pain genuineness. Participants (n = 90) observed video depictions of chronic pain patients performing a physically challenging task and were asked to make inferences of pain intensity and pain genuineness. Analyses indicated that observers relied on judgmental heuristics and pain behaviors both when making inferences about pain intensity and when making inferences about pain genuineness. Follow-up analyses, however, revealed that judgmental heuristics (eg, gender stereotypes) were significantly less utilized when observers made inferences about pain genuineness than when observers made inferences about pain intensity. When observers made inferences about pain genuineness, analyses indicated that patients' facial pain behaviors became the most important source of information. Taken together, these findings suggest that observers who are asked to make inferences about the genuineness of others' pain are likely to reduce their reliance on judgmental heuristics in favor of more controlled and thoughtful inferential processes characterized by detailed processing of behavioral information, particularly others' facial pain behaviors. PERSPECTIVE: The current study provides new insights into the processes that are involved in observers' inferences about pain intensity and pain genuineness. These inferences play an important role in treatment decisions and advances in this domain could ultimately contribute to more effective management of the challenges facing patients with pain-related disorders.

Pain, perceived injustice and the persistence of post-traumatic stress symptoms during the course of rehabilitation for whiplash injuries.

The present study assessed the role of pain and pain-related psychological variables in the persistence of post-traumatic stress symptoms following whiplash injury. Individuals (N=112) with whiplash injuries who had been admitted to a standardized multidisciplinary rehabilitation program were asked to complete measures of pain, post-traumatic stress symptoms, physical function and pain-related psychological variables at three different points during their treatment program. The findings are consistent with previous research showing that indicators of injury severity such as pain, reduced function and disability, and scores on pain-related psychological were associated with more severe post-traumatic stress symptoms in individuals with whiplash injuries. Contrary to expectations, indicators of pain severity did not contribute to the persistence of post-traumatic stress symptoms. Univariate analyses revealed that self-reported disability, pain catastrophizing and perceived injustice were significant determinants of the persistence of post-traumatic stress symptoms. In multivariate analyses, only perceived injustice emerged as a unique predictor of the persistence of post-traumatic stress symptoms. The results suggest that early adequate management of pain symptoms and disability consequent to whiplash injury might reduce the severity of post-traumatic stress symptoms. The development of effective intervention techniques for targeting perceptions of injustice might be important for promoting recovery of post-traumatic stress symptoms consequent to whiplash injury.

Darwin and emotion expression.

In his book The Expression of the Emotions in Man and Animals, Charles Darwin (1872/1965) defended the argument that emotion expressions are evolved and adaptive (at least at some point in the past) and serve an important communicative function. The ideas he developed in his book had an important impact on the field and spawned rich domains of inquiry. This article presents Darwin's three principles in this area and then discusses some of the research topics that developed out of his theoretical vision. In particular, the focus is on five issues--(a) the question of what emotion expressions express, (b) the notion of basic emotions, (c) the universality of emotion expressions, (d) the question of emotion prototypes, and (e) the issue of animal emotions--all of which trace their roots to Darwin's discussion of his first two principles.