Atherosclerosis and Lipoproteins |
From the Department of Psychology (L.K.-J., N.R.), University of Helsinki, Helsinki; and the Department of Medicine (J.V.), University of Turku, Turku, Finland.
Correspondence to Niklas Ravaja, Department of Psychology, PO Box 4, FIN-00014, University of Helsinki, Helsinki, Finland. E-mail Niklas.Ravaja{at}Helsinki.FI
| Abstract |
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Key Words: temperament cluster analysis coronary heart disease risk insulin resistance syndrome
| Introduction |
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There are various definitions of the concept of temperament in the research literature. The most general definition is that temperament consists of biologically rooted individual differences in behavior tendencies that are present early in life and are relatively stable across time and situations5 ; temperament highlights a person's behavioral style, ie, the how of behavior.6 7 Temperament is also the biologically rooted core of an individual's personality; personality develops from the interaction between innate temperament and experience.6 Temperament has been shown to be inherited, at least in part; on average, slightly <50% of the variance in temperament traits appears to be explainable by referring to the genetic factor, although the heritability estimates for different dimensions vary considerably.8 9 10 11 12 Although temperament is subject to changes caused by maturation and environmental factors, its dimensions have been shown to be rather stable,13 14 even from childhood to adulthood.15 16
Temperament has its origin in brain structure, and individual differences in temperament are attributable to differences in neural and physiological functions (eg, see Reference 1717 ). In the past decade, evidence of the moderating effect, even of the cause-effect relationship, of temperament on stress-induced physiological arousal has increased markedly. This relationship has been demonstrated in both major neuroendocrine stress systems, ie, the sympathetic18 19 20 21 22 23 24 25 and adrenocortical (usually cortisol level18 21 25 26 27 28 29 30 ) systems. Most studies have, however, been carried out among children, even toddlers, and the stressors used have been age-appropriate, eg, separation from the mother and joining a new peer group. The findings are thus not applicable to adults for assessing daily life stress: stress in the workplace, for instance.
The role of temperament-related behavioral characteristics, like type A behavior and hostility, or temperament-rooted personality traits, like introversion-extraversion, in the pathogenesis of cardiovascular disease has been widely investigated. However, innate temperament as such, ie, with the use of temperament theories and their operationalizations, has not usually been treated as a behavioral risk factor for somatic diseases. This is surprising, because the well-known individual variation in sympathetic-adrenomedullary and hypothalamic-pituitary-adrenocortical activation by brief psychological stressors might help us to understand why daily irritations and stressors have greater health consequences for some individuals than for others. Inherited temperament might be especially relevant to the etiology of coronary heart disease (CHD), in which the roles of stress and sympathetic-adrenal (SA) and hypothalamic-pituitary-adrenal (HPA) axis activity are well documented.3 This study was undertaken with that in mind. We were interested in (1) identifying naturally occurring temperament profiles in a population-based sample of young adult men and (2) examining the relationship of those profiles with the important physiological CHD risk factors included in the metabolic cardiovascular syndrome measured during adolescence and young adulthood.
The metabolic cardiovascular syndromealso called the insulin resistance syndrome (IRS)is a plurimetabolic disorder consisting of insulin resistance, compensatory hyperinsulinemia, glucose intolerance, an increased concentration of serum triglycerides (TGs), a decreased concentration of serum HDL cholesterol (HDL-C), and hypertension.31 32 33 Cellular resistance to insulin-stimulated glucose uptake has been postulated as the primary metabolic defect responsible for the other manifestations of the syndrome.33 Even though CHD usually manifests itself in middle age or later, the atherosclerotic process underlying it originates in childhood and adolescence, and coronary risk factors are already present then.34 Thus, it is important to understand the pathogenic processes associated with the development of high levels of metabolic CHD risk factors, even in the young. The metabolic cardiovascular syndrome was considered to be of special relevance here, because the role of stress and stress-induced arousal of the SA and HPA axes in the development of this syndrome is widely accepted.35 36 37 38
Cloninger's biosocial temperament theory was adopted for assessing temperament.39 40 According to this theory, 3 genetically homogeneous and independent temperament dimensions exist, each of which is 50% to 65% inheritable.10 These 3 dimensions are defined in terms of individual differences in the sensitivity to specific environmental stimuli and the behavioral responses to those stimuli, and they reflect variation in 3 brain systems; ie, each dimension is associated with a specific neurotransmitter.39 40 The temperament dimensions are novelty seeking (NS), harm avoidance (HA), and reward dependence (RD39 40 ). NS reflects behavioral activation; it is a heritable tendency toward frequent exploratory activity and intense excitement in response to novel stimuli, as well as active avoidance of monotony. It is correlated with low basal dopaminergic activity. HA is connected with behavioral inhibition and high serotonergic activity. It reflects a heritable tendency to respond intensely to aversive stimuli and to learn to avoid punishment, novelty, and frustrating nonreward. RD is connected with behavioral maintenance. It reflects a heritable tendency to respond intensely to signals of reward (particularly verbal signals of social approval and succor) and to maintain or resist extinction of behavior that has previously been associated with rewards or relief from punishment. It is correlated with low basal noradrenergic activity. Persistence (P) later emerged as a distinct fourth dimension, although it was originally thought to be a component of RD and was measured in terms of perseverance despite frustration and fatigue.
Previous literature does not give a basis for specific hypotheses. There is, however, evidence for an association of NS with some behavioral risk factors of CHD, such as smoking41 and alcohol abuse.41 42 Also, HA is close to the concept of social inhibition, which in turn has been associated with social aloneness and high reactivity of the SA and HPA axes during social challenge,23 whereas RD includes aspects totally opposite to the traditional psychological CHD risk factors.39 40 That being so, HA and NS were expected to be related to high levels of metabolic CHD risk factors whereas RD was suggested to be a protective factor.
| Methods |
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The participants in the current study were the 3 oldest male age cohorts of the CRYF study who, at the baseline of the present study (year 0, n=866) were 12 to 18 years of age and consequently were 15 to 21, 18 to 24, and 29 to 35 years of age during the first (year-3 follow-up, n=645), second (year-6 follow-up, n=565), and third (year-17 follow-up, n=400) follow-up examinations in 1983, 1986, and 1997, respectively. At years 0, 3, 6, and 17, complete data were available from 190 participants. Despite the high dropout rate, there was no systematic bias in attrition.
Measures
Temperament
At year 17, temperament of the subjects was measured by
Cloninger's Temperament and Character Inventory (TCI), a 240-item,
self-administered questionnaire.40 The TCI was designed to
assess the 4 temperament dimensions and 3 character dimensions as
defined by Cloninger's theory.39 40 In the present
study, however, only the 4 temperament scales comprising 107 items were
used. The temperament dimensions addressed by the TCI are as follows:
(1) NS (4 subscales: exploratory excitability versus stoic rigidity,
impulsiveness versus reflection, extravagance versus reserve, and
disorderliness versus regimentation), (2) HA (4 subscales: anticipatory
worry and pessimism versus uninhibited optimism, fear of uncertainty,
shyness with strangers, and fatigability and asthenia), (3) RD (3
subscales: sentimentality, attachment, and dependence), and (4)
P.
Examples of NS items from the TCI include "I'm slow to get excited about new ideas" (reverse scored), "I think in detail before deciding" (reverse scored), "I'm better at saving money than most" (reverse scored), and "I do things spontaneously." Examples of HA items include "I'm confident that things will go well" (reverse scored), "I get tense and worried in unfamiliar situations," "I avoid meeting strangers," and "I have less energy than most." The RD scale consists of items such as "I'm strongly moved by sentimental appeals," "I don't open up much even with friends" (reverse scored), and "Others think I am too independent" (reverse scored). The P scale consists of items such as "I often push myself to exhaustion," "I work long after others give up," and "I am satisfied with my accomplishments, and have little desire to do better" (reverse scored). Items of each temperament scale were added up to yield total scores of NS, HA, RD, and P.
The content, construct, and predictive validity as well as reliability of the TCI scales in a Finnish sample have previously been shown to be acceptably high.44
Metabolic Coronary Risk Factors
The following measurements relevant for the
IRS31 32 were obtained for all participants: serum
insulin, serum HDL-C, serum TGs, systolic blood pressure (SBP),
weight (in kilograms), height (in meters), and subscapular skinfold
thickness (SSF). These physiological
parameters were measured at years 0, 3, and 6.
BP was measured with a standard mercury gravity sphygmomanometer on the right arm, after a rest of at least 3 minutes, to the nearest even figure.45 SSF was measured by Harpenden calipers (Holtain and Bull-British Indicators instruments) to the nearest 0.2-mm reading. SSF was used as an index of upper-body subcutaneous fat distribution. Height was measured by Seca anthropometer and weight by Seca scales. Body-mass index (BMI) was calculated as weight divided by height squared. The procedures for anthropometric measurements have been described in detail elsewhere.46
After an overnight fast, venous blood samples were taken from the right antecubital vein of recumbent subjects. Serum insulin was measured by a modification of the immunoassay method of Herbert et al47 in the Research Laboratory of the Department of Pediatrics, University of Oulu, Oulu, Finland. The intra-assay CV was 1.6% and the interassay CV 8.4%, the sensitivity of the assay being 0.5 mU/L. All lipid determinations were done in duplicate, with standard enzymatic methods used for serum cholesterol (Boehringer CHOD-PAP48 ) and TGs (Boehringer49 ) in the laboratory of the Rehabilitation Research Center of the Social Insurance Institution, Turku, Finland. This laboratory continuously cross-checks the lipid determinations with the World Health Organization laboratory in Prague, Czech Republic. Serum HDL-C concentrations were measured from the serum supernatant after precipitation of VLDL and LDL lipoproteins with dextran sulfate 500 000.50 All analyses were performed as simultaneously as possible. The averaged within-run and between-run CVs were 1.7% and 3.8% for HDL-C and 2.6% and 4.4% for serum TGs. A more detailed description of our assessment protocol has been reported elsewhere.51
Design of the Study
In principle, given the hypothesis that temperament may
influence the IRS, temperament should have been measured before, or at
the same time as, the metabolic CHD risk factors. In the
present study, however, temperament (independent variable) was
measured after the metabolic risk factors (dependent
variables); that being so, the design of the study might be
regarded as inappropriate. Several facts support the view that the
present study design is warranted: (1) As mentioned above, the
atherosclerotic process underlying CHD originates in childhood and
adolescence, and coronary risk factors are already present
then.34 In addition, both individual risk factors making
up the IRS and the clustering of these risk factors show considerable
tracking* from childhood to adulthood.52 (2)
The Cloninger temperament model is likely to be particularly relevant
in the present connection, given that it focuses on the underlying
neurobiological basis of temperament; such a focus enables more
specific suggestions as to the mechanisms through which temperament is
likely to influence metabolic CHD risk factors. In
addition, no childhood version of the TCI is available; although
temperament scales for use in childhood are available, they are based
on other temperament models. (3) Importantly, in regard to Cloninger's
model, the number and structure of temperament dimensions based on
teacher ratings throughout childhood have been shown to be similar to
those observed in self-reports by adults53 ; ie, these
temperament dimensions exhibit continuity from childhood to adulthood.
Of course, although the structure of temperament (conceptually similar
temperament factors) and the neurophysiological
basis of the different dimensions remain the same from age to age, the
behavioral manifestations of these dimensions may change, at least
slightly, across ages (ie, heterotypic continuity). (4) If an
association is found between adulthood temperament dimensions (the
focus of the present study) and adolescence metabolic
CHD risk factors, this result is in effect a strong argument for the
stability of the temperament dimensions measured and the robustness of
the association studied.
Statistical Analyses
Temperament Patterns
In the person-oriented pattern approach, each subject's
complete profile of available data are considered, and individuals are
grouped together on the basis of similarities in these
profiles.54 This approach deals with data and presents
them in terms directly applicable to individuals, in contrast to the
traditional regression models, which yield results referring to "the
average person," who simply does not exist.54 Cluster
analytic techniques are 1 useful method of pattern description.
Cluster analysis served to identify subgroups of individuals with similar patterns or profiles on the 4 temperament variables. The cluster analysis was performed with SLEIPNER, a statistical package for pattern-oriented analyses.55 Before the cluster analysis, we identified a residue of multivariate outliers with the RESIDUE module of SLEIPNER and removed those subjects who belonged to the residue.56 For the cluster analysis, raw scores for each temperament variable were standardized as z scores. To perform a preliminary classification, the standardized scores for NS, HA, RD, and P were entered into SLEIPNER'S CLUSTER program with use of the Ward clustering algorithm. This is an agglomerative hierarchical clustering technique that uses the squared euclidean distance to determine the similarity between subjects' profiles on the variables. The Ward algorithm iteratively joins those clusters, the fusion of which minimizes an increase in the within-cluster or error sum of squares (error SS) while maximizing the between-cluster sum of squares. (The error SS is defined as the sum of the distances from each individual's profile to the centroid of its parent cluster.) Subjects whose profiles have similar elevations and patterns will consequently be grouped together.
A major issue with cluster analysis is how to select the number of clusters. Unfortunately, no standard, objective selection procedure exists. However, the optimal solution for the number of clusters to retain in a cluster analysis can be based on the substantial increase in the error SS associated with a particular number of clusters compared with the error SS associated with the previous iteration. Small step-by-step increases in the error SS indicate additional clustering of cases and/or clusters of relatively homogeneous characteristics. The merging of cases and/or clusters of dissimilar characteristics is indicated by a large increase in the error SS. Thus, the optimum number of clusters is reached at the stage of the clustering process preceding the substantial increase in the error SS. It has been suggested, however, that it is probably best to compute a number of different cluster solutions (eg, 3, 4, and 5) and then decide among the alternative solutions by using a priori criteria, practical judgment, common sense, or theoretical foundations.57
After the preliminary classification, the subjects were relocated to the clusters by using SLEIPNER'S RELOCATE module. RELOCATE starts from an initial classification and moves cases from 1 cluster to another if that reassignment leads to a reduction in the total error SS of the cluster solution. This produces more homogeneous clusters.
IRS Factor
Physiological parameters, ie,
serum insulin, HDL-C, TGs, SBP, BMI, and SSF, were subjected to
principal-components analysis. The resulting IRS factor is
described under Results.
Relationship of Temperament Patterns to Metabolic
Coronary Risk Factors
Differences in metabolic risk factors, ie, serum
insulin, HDL-C, TGs, SBP, BMI, SSF, and IRS factor, between the groups
(temperament clusters) were analyzed by ANCOVA with age as the
covariate (see also the
footnote).
| Results |
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IRS Factor
Principal-components analysis of the
physiological parameters resulted in 1
factor that accounted for 44%, 42%, and 43% of the total variance of
these parameters at years 0, 3, and 6, respectively. This
factor was named the IRS factor and is presented in Table 1
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Relationship of Temperament Patterns to Metabolic
Coronary Risk Factors
Table 2
shows the mean values for
the physiological parameters in the
study group at years 0, 3, and 6 separately for the age cohorts. Figure 2
is a graphical representation
of the relationship between year-17 temperament clusters and
metabolic coronary risk factors measured at years
0, 3, and 6. The Figure
reveals that with some minor exceptions, the
level for metabolic risk factors at years 0, 3, and 6 was
highest among those subjects belonging to cluster 1 (ie, those
characterized by high P and RD, low HA, and an average level of NS) and
lowest among subjects belonging to cluster 4 (ie, those characterized
by high HA, low P and NS, and an average level of RD). The ANCOVA
indicated that of the individual physiological
parameters, SSF differentiated the temperamental clusters
(groups) statistically significantly at years 3 and 6: F(3177)=2.94 and
3.27, P=0.035 and 0.022, respectively, and marginally
significantly at year 0: F(3177)=2.61, P=0.053. The ANCOVA
also showed that the temperament clusters were significantly
differentiated by insulin at year 3: F(3177)=6.09, P=0.001.
In addition, the temperament clusters were marginally significantly
differentiated by year-3 BMI, year-6 TGs, and year-6 HDL-C: F=2.23,
2.25, and 2.31, and P=0.087, 0.085, and 0.078, respectively.
In addition, the IRS factor score significantly differentiated the
temperament clusters at years 0, 3, and 6: F(3177)=2.84, 5.69, and
3.17, and P=0.040, 0.001, and 0.026,
respectively.
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| Discussion |
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Individuals with high P express perseverance despite frustration and fatigue.40 Individuals with high RD, in turn, are described as being sympathetic persons eager to help and please others and highly dependent on emotional support and intimacy with others, even vulnerable because of their openness. They are oversocialized, with an excessive tendency to conform to peer pressures, and highly sensitive to social cues of approval/praise and rejection from others, leading to excessive reward-seeking behaviors such as overeating or overworking in response to social rejection or frustration. As workers, like those manifesting high P, they are confident, hard-working, persistent, ambitious, heroic, and forceful overachievers who frequently push themselves to exhaustion.39
For a naturally occurring cluster, being a combination of different traits with balancing effects, a low level of HA is also significant. Low HA is characterized by a lack of inhibition and of appropriate caution, even when the situation requires them; by activities with a risk of physical injury; and by high energy levels with quick recovery from exertion, stress, and minor illness and rapid adaptation to changes in familiar routines. If low-HA persons are at the same time low in NS, as here, they will usually be cheerful, boastful, and overconfident.39
The "at-risk person" portrayed in the current study appears to challenge the traditional coronary-prone personality, at least to some extent. Actually, most of the widely documented coronary-prone characteristics were not only lacking in the present high-risk person, but also many of them, such as cynicism, pessimism, anticipatory worrying, tenseness, and slow recovery from stress, were determinants of a low-risk profile. Of course, the risk factor status of hostility, for example, is so well established59 60 that we are not trying to question previous findings; it is noteworthy, however, that the most recent meta-analytic review suggested that the relationship between the cognitive-experiential measures of hostility and CHD is rather modest (weighted mean r=0.0860 ). Although this discrepancy may be difficult to explain, it should be recognized that the present temperament dimensions reflecting individual differences in associative learning in response to novelty, danger or punishment, and reward (ie, preconceptual biases in learning40 ) are not identical to any of the personality traits traditionally examined in relation to CHD (eg, hostility), the development of which involves insight learning or reorganization of self-concepts.
The present findings are, however, in line with those studies suggesting that ambitious overachieving and the resulting exhaustion are risk factors for CHD: the present high-risk person is described as an effort-oriented one who easily pushes himself to exhaustion. Van Doornen and Van Blokland61 have, for instance, suggested that a combination of ambitious striving for achievement and exhaustion might be the core concept of coronary-prone behavior. The pathogenic role of exhaustion that people attribute to overwork is in turn well documented in several CHD studies by Appels and coworkers.62 63 64
Temperament, though apparent at the behavioral level, also refers to the neurobiological and genetic determinants of behavior; so far as Cloninger's theory is concerned, it refers to the neurobiology of motivation and learning.39 As mentioned before, Cloninger's 3 temperament dimensions (with P added later) reflect variation in the 3 brain systems and the biogenetic predispositions to stimulus-response characteristics. RD, 1 of the characteristics of the risk profile, is responsible for the maintenance of behavior that has previously been socially appreciated or rewarded (overworking or overresponsibility, for instance) despite frustration of one's efforts; relevant stimuli are conditioned signals of rewards, especially social cues and social approval. In other words, signals of social approval (secondary reinforcements) are of such a strong influence that behavior (overworking) is likely to be maintained despite lack of actual benefits and rewards (primary reinforcements).
Norepinephrine is the principal monoamine neuromodulator for the system. Norepinephrine plays a critical role in the learning and memory of new paired associations. Individuals with reduced basal activity in the dorsal noradrenergic bundle, and hence greater sensitivity to norepinephrine, are expected to respond to signals of social sentiment and to persist in reward-seeking behavior, even when frustrated, whereas those with lower sensitivity to norepinephrine quickly cease activity that is no longer gratifying.39
Regarding the "risk temperament" discovered herein and the role of achievement and effort orientation in it, catecholamine secretion has repeatedly been shown to be related to performance or striving for achievement.65 66 Superior performance has also been associated with an increase in norepinephrine, reflecting a challenge to perform well.66 67 Release of catecholamines also occurs in stress situations, especially during acute stress.68 As to the potential role of the SA system in the genesis of the IRS, it has been shown, for example, that catecholamines exert marked insulin resistance in humans.69 70 Also, Lembo and coworkers71 have recently demonstrated that an acute increase in skeletal muscle noradrenergic activity, as measured by norepinephrine outflow, is able to antagonize insulin-stimulated muscle glucose disposal. In addition to the metabolic hormonal effects, chronic sympathetic overactivity may also induce insulin resistance by inducing hemodynamic changes.70 72 Of course, given that a combination of high RD and P is likely to expose an individual to chronic work stress, for example, an increased release of cortisol may also be important; cortisol is also known to exert marked insulin resistance.35 36
Although this study's associations between temperament and physiological CHD risk factors did not always reach statistical significance, they did show a high consistency across all physiological parameters. A relationship to the whole IRS was most evident, and single associations with insulin and SSF were also significant. Compensatory hyperinsulinemia is the final common denominator ultimately responsible for the other changes of the IRS, and insulin itself is also a major risk factor for the development of CHD.32 73
The present empirical associations between physiological risk factors and temperament based on Cloninger's concept support the view that there is a neurobiological basis for that temperament, and the well-known connections between adrenergic responsivity and the pathogenesis of coronary atherosclerosis allow construction of a model in which all pieces fit. The relevance of the current findings is in part dependent on the continuity and stability of the temperament dimensions measured. As mentioned before, the temperament dimensions included in the Cloninger temperament model have previously been shown to exhibit continuity from childhood to adulthood53 ; of course, it is possible that the manifestations of these dimensions change slightly from young adulthood to later ages, at which time myocardial infarction may occur.
Some methodological limitations should also be recognized. First, there
are problems when one attempts to measure temperament by means of a
questionnaire. For example, although Cloninger's temperament
dimensions by definition reflect heritable biases in information
processing by the perceptual memory system40 and the
heritability estimates for the TCI temperament dimensions are higher
than is often the case,10 substantial proportions of the
variance of these dimensions are determined by other than genetic
factors. Second, if a different method of cluster analysis had
been used, the behavioral clusters would have been slightly different.
Of course, the results must also be cross-validated in other samples,
although there is some evidence that the present cluster solution
has some generality (see the
footnote). Thus, the present
findings should not be overinterpreted.
To summarize, the present study demonstrated that a temperament profile characterized by a high level of P and RD, an average level of NS, and a low level of HA is related to a high level of physiological CHD risk factors included in the IRS. This association may be predicated on several factors: these temperament factors may (1) increase exposure to stressors; (2) affect appraisal of stressors, thereby modulating physiological responses to stress; and (3) be markers of the same biological processes that underlie the development of high levels of metabolic CHD risk factors. The findings suggest that the temperament profile in question may predispose an individual to the development of the IRS and CHD.
| Acknowledgments |
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Received November 17, 1998; accepted January 8, 1999.
| References |
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