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Serial Killers and Intelligence Levels: Variability, Patterns, and Motivations to Kill.

Serial Killers

As cited by Leary, Southard, Hill, and Ashman (2017), the term "serial killer" was first used in the 1970s when FBI Special Agent Robert Ressler coined the term "serial murderer" during the notorious Son of Sam killings (Miller, 2014). This definition established serial murder as distinctly different from traditional murders. The Federal Bureau of Investigation defines serial murder as "the unlawful killing of two or more victims by the same offender in separate events" (Federal Bureau of Investigation, 2008, p. 9). All serial killers in the database used for the present study met these criteria.

The terms "psychopath" and "serial killer" are not synonymous. However, the term "criminal psychopath" is reserved for those psychopaths who demonstrate a wide range of persistent and antisocial behavior (Bartol & Bartol, 2014); antisocial personality disorder encompasses criminally psychopathic behaviors marked by a lifelong pattern of the manipulation and violation of others' rights, as defined in the Diagnostic and Statistical Manual of Mental Disorders V (American Psychiatric Association, 2013). Serial killers are criminal psychopaths and represent an extreme variant on the spectrum of antisocial personality disorder (Leary et al., 2017).

Intelligence Level

Early theorists Pinel (1801), Rush (1812), and Krafft-Ebing (1886) observed and theoretically hypothesized that those we now recognize as criminal psychopaths (e.g. serial killers), possessed high intelligence in conjunction with antisocial tendencies (Vitacco, Neumann, & Wodushek, 2008). In 1941, the eminent psychiatrist, Cleckley (1941), in his classic, provocative, and seminal book The Mask of Sanity, portrayed criminal psychopaths as intelligent, glib, egocentric, and charming. Since then, sentiments echoed by other eminent researchers (Bullard, 1941; Douglas, Burgess, Burgess, & Ressler, 1992; Hankoff, 1961; Knoll, 2006; Macdonald, 1966) have reinforced perceptions of above-average to superior intelligence among criminal psychopaths who serially kill. However, empirical studies have failed to consistently demonstrate differences in intelligence between psychopaths, in general, and nonpsychopaths (Harpur, Hare, & Hakstian, 1989; Johansson & Kerr, 2005). As noted by Hare (2003), near zero correlations exist between intelligence and psychopathy, and this relationship has yet to be convincingly demonstrated (Johansson & Kerr, 2005).

Generic research on psychopaths versus serial killers, an extreme variant (subgroup) within this construct, has preempted an opportunity to evaluate specific relationships between serial killers, per se, and intelligence level. A paucity of case studies, archived clinical descriptions (Krafft-Ebing, 1886; MacDonald, 1963), and FBI assessments (Douglas, Ressler, Burgess, & Hartman, 1986) have fueled perceptions that "organized" (as opposed to "disorganized") serial killers are, in fact, highly intelligent; however, until recently, more comprehensive data e.g., a wider range of recorded intelligence quotients [IQs] to validate this relationship between serial killers and intelligence were virtually nonexistent. Consequently, despite historical anecdotes, previous observations, and sparse clinical assessments, the association between intelligence level and serial killers remained obscure. Additional factors contributing to this ambiguity include variable measures of intelligence that are not aligned to theory (Salekin, Neumann, Leistico, & Zalot, 2004) and data from a small sample of highly profiled serial killers (those who are the most vivid, memorable, and coincidentally, more intelligent) who are not necessarily typical of serial killers in general. Exacerbated by the Hollywood Effect (Canter & Youngs, 2003), infamous predators (e.g., Ted Bundy, Ted Kaczynski) who are also intellectually gifted (e.g., IQ recorded at genius level) have been inaccurately placed on a pedestal by the media, who portray them as prototypical serial killers. Similarly, the popularization of serial murder as epitomized by the fictional character of Hannibal Lecter in the film The Silence of the Lambs (DeLisi, Vaughn, Beaver, & Wright, 2010) has also reinforced illusory correlations between high intelligence and serial killing without solid empirical evidence to either accept or reject this premise.

Because previous research regarding the relationship of intelligence to serial killers had been limited and somewhat inconclusive, access to updated, comprehensive, versions of the Radford/Florida Gulf Coast University (FGCU) Serial Killer Database (Aamodt, 2013; Aamodt, Leary, & Southard, 2019) provided a unique opportunity to extract the IQs of up to 303 serial killers. We can now contrast the distribution of IQ levels among serial killers to normal population parameters and elucidate more valid associations among intelligence, methodology (organized vs. disorganized), and motivations (financial, enjoyment) to serially kill.

Methodology (Organized/Disorganized)

The most useful typology by which to group serial killers methodologically as either organized or disorganized is in Douglas et al.'s Crime Classification Manual (Canter, Alison, Alison, & Wentink, 2004; Knoll, 2006). A third category within this mixed typology includes those perpetrators not categorically bound as either organized or disorganized. To not obfuscate distinct characteristics associated with organized or disorganized killers, those exhibiting mixed characteristics were excluded from this study.

"Like a fingerprint," a crime scene is utilized to identify a perpetrator while also providing behavioral and personality characteristics as a function of crime scene evidence (Canter et al., 2004; Douglas et al., 1986). Generated from casual interviews with 36 incarcerated serial killers, information was gleaned from examination of the crime scene, victims, and forensic reports (Douglas et al., 1992). Over time, additional exemplars validated the template by which to categorize a killer as primarily organized or disorganized based on such crime scene characteristics. This organized/disorganized typology became widely utilized and accepted as a conceptual tool at the heart of "offender profiling" (Jackson & Bekerian, 1997); what this approach does, and does very well, is examine crime scene characteristics and, based on methodology, classify the perpetrator accordingly (Holmes & Holmes, 1998).

Without providing a complete exegesis of differences between each, the following synopsis contrasts key characteristics of organized versus disorganized serial killers. The organized serial killer can appear socially competent, is a meticulous planner, a bully, somewhat orderly, an effective predator, and intelligent (Canter et al., 2004; Douglas et al., 1992; Knoll, 2006). Conversely, disorganized serial killers demonstrate poor verbal and social skills; lack of planning before, during, and following a crime; a chaotic approach to execution (e.g., random, sloppy), and presumably are not as intelligent as organized serial killers (Canter et al., 2004; Douglas et al., 1992).

Because previous data linking intelligence level with a methodological approach to serial killing was somewhat limited, the Radford/FGCU Serial Killer Database (Aamodt, 2013; Aamodt et al., 2019), a source of updated, comprehensive information, provided a unique opportunity to extract information pertaining to both organized and disorganized serial killers; the anticipated outcome (this venture) should elucidate clearer, more valid associations between IQs and methodology (organized vs. disorganized) based on crime scene criteria.

Primary Motivation (Financial, Enjoyment)

As cited by Aamodt et al. (2019), the most frequently documented primary motivations to serially kill were either enjoyment of killing (36%) or financial gain (30.2%). Closer scrutiny revealed that for Enjoyment, the most prominent sub-categories included EnjoymentRape (70.5%) and Enjoyment-No rape (20.5%). In order to elucidate robust relationships, only serial killer data from the most frequently documented categories (Financial gain, Enjoyment-Rape, Enjoyment-No rape) were included for evaluative purposes. Perpetrators with infrequently coded primary motivations to serially kill (e.g., psychosis, cult/gang memberships, multiple motivations) were excluded from this aspect of the study.

Intelligence Level, Methodology, and Motivations

IQs are roughly symmetrical, bell-shaped, and normally distributed (Aron, Aron, & Coups, 2007); the mean, median, and mode are 100, with a standard deviation of 15. Logically extended, if the recorded IQs of serial killers modeled the normal population relative to intelligence, descriptive statistics, and critical measures of dispersion (variance and standard deviation) would be similar for both groups. A bell-shaped distribution (normal population curve) approximates the following dispersion of IQs within each classification (Wechsler, 1997); see Table 1:

Distribution of IQs and measures of central tendency that vary from normal population parameters may reflect a concentration of intelligence level among serial killers within one or more of the above classifications. For example, a concentration of serial killer IQs in the Extremely Low category might dispel original misconceptions that serial killers are significantly more intelligent than the norm; it may also reflect a skewed sample based on availability, not reflective of serial killer intelligence in general. Using the Radford/FGCU Serial Killer Database, the following query was generated on an exploratory basis to research the nature of IQ distributions within our serial killer sample relative to the normal population:

Research question #1: Is there a difference in measures of central tendency (mean, median, and mode) and dispersion (variance) between the IQs of serial killers and the normal population? If so, how can this be explained?

Because previous data linking intelligence level with crime scene methodological approach (organized/disorganized) was somewhat limited, the opportunity to assess this relationship from the broader spectrum of serial killers in the Radford/FGCU database led to the following exploratory investigation:

Research question #2: Is there an association between IQ level (e.g., high vs. low) relative to the methodology (organized vs. disorganized) used by serial killers?

In order to investigate the relationship between intelligence and drivers to commit acts of serial murder, a recent compilation of primary motivations from the Radford/FGCU database enabled us to extrapolate this association. The following query addressed this issue:

Research question #3: Are there differences in intelligence level and the primary motivations of Financial, Enjoyment-Rape, or EnjoymentNo rape among serial killers?



Three hundred and three participants were selected from 5,014 serial killers included in the Radford/FGCU database (Aamodt, 2016; Aamodt et al., 2019). These participants were the only serial killers in the database with recorded IQs. All committed crimes between 1950 and the present. Demographically, 291 participants were male, 12 were female, 180 were Caucasian, 103 were Black, 13 were Hispanic, 4 were Asian, and 3 were Native American. Procedure

An IQ score for each serial killer (n = 303) was retrieved from the database; 31% had more than one recorded IQ score. In all cases, the highest IQ was selected to provide a more accurate measure of intelligence; individuals can fake a lower IQ in order to preempt execution (Atkins v. Virginia, 2002) but they cannot fake being smart or having a higher IQ.

Measures of central tendency (mean, median, mode) and dispersion (e.g., variance) of serial killer IQs were statistically compared to those of normal population parameters; a binomial test compared the percentage distribution of Very Superior and Extremely Low IQs by category for serial killers in contrast to the theoretical normal curve (Wechsler, 1997).

Potential relationships between serial killer IQs and crime scene methodology (disorganized or organized crime scene characteristics) were then examined for those who met the inclusion criteria noted above (coded as either organized or disorganized, only). A further, more detailed analysis segregated serial killers with Very Superior IQs (130 or greater) versus Extremely Low IQs (< 70) to elucidate a clearer distinction between very high versus very low intelligence relative to differences in crime scene methodology (organized/disorganized). Finally, an extrapolation of the most frequent primary motivations to serially kill (enjoyment, financial) to that of IQ level was performed for those who met this inclusion criteria. A subsequent, more detailed analysis examined potential relationships between IQ level and the primary motivations of Enjoyment-Rape versus Enjoyment-No rape.

Whenever possible, multiple sources were used to validate each piece of information. In cases of conflicting information, the more official source was used, such as court documents versus media-based sources (Aamodt et al., 2019).


A series of statistical tests analyzed key relationships; the scope of tests utilized included a one sample t-test (Table 2), chi-square test for single variance (Table 3), binomial test analyses (Table 4), and independent samples t-tests (Table 5).

One Sample t-Test Analysis

A one sample t-test was performed to evaluate differences between the average IQ of serial killers and the normal adult population. A criteria alpha level of.05 was established. The result of the one sample t-test was statistically significant at the p<.05 level. The mean IQ of serial killers (N = 303) was M = 93.21 versus the normal adult population of M = 100 (Wechsler, 1997). The actual test result was t (302) = -4.695, p<.01.

Chi-Square Test for Single Variance Analysis

A chi-square test for single variance was performed to assess differences in variation within each discrete population; in other words, does the variation in IQ level within the serial killer population approximate the variation within the normal population (225)? An alpha level of.05 was selected as the criteria for statistical significance. The result of the chi-square single variance test (serial killer IQ) was statistically significant at p <.01, in contrast to the normal population IQ variance (225); [chi square] (302) = 850.707.

Binomial Test Analysis

In order to explain the unique dispersion of serial killer IQs and to further pinpoint the contrast with that of normal adult populations (see Table 4), two binomial tests were conducted. For exploratory purposes, Very Superior and Extremely Low IQ classifications were evaluated relative to both expected and observed percentage frequencies. Within the normal population, IQs at either extreme are infrequent and represent around 2.2% of the population (Wechsler, 1997). At each categorical extreme, a binomial analysis indicated a statistically significant difference (p<.0005). In fact, at the Very Superior level, perpetrators were almost five times more likely than the general population to have a Very Superior IQ and seven times more likely to have an extremely low IQ. The calculated effect size, using the arcsine transformation methodology for proportions, between Very Superior IQ levels of serial killers (observed) versus the normal population (expected) was h= 0.37, small to medium, yet a noteworthy effect size. For those with Extremely Low IQs (observed) versus the normal population (expected) a medium effect size, h = 0.50 (Cohen, 1988) was calculated. An independent sample t-test that was subsequently conducted further elucidated the association between extremely high and extremely low IQ levels and organized versus disorganized crime scene characteristics.

Independent Samples i-Test Analyses

A series of four independent sample t-tests were performed to determine any relationships of the IQs of serial killers to crime scene methodology (organized, disorganized) and key motivations to serially kill (financial vs. enjoyment, enjoyment with rape vs. non-rape) to serially kill. An additional, higher impact analysis was conducted to further segregate the relationship between IQ levels [greater than or equal to] 130 and <70 relative to organized versus disorganized methodology. An alpha level of .05 was chosen to indicate whether differences were statistically significant.

The first independent samples t-test to assess differences between intelligence level (high versus low) and methodology (organized vs. disorganized) among serial killers was significant at the p<.050 level (p =.002). The mean IQ for the organized group (N = 139) was higher (M = 99.93, SD = 26.44) in contrast to the disorganized group (N = 78, M = 89.60, SD = 21.06). Since the assumption of homogeneity of variance was violated (F = 8.99, p =.003), the degrees of freedom were adjusted from 215 to 190 using the Welch-Satterthwaite method. The end result was t(190) = 3.16, p<.050, (p =.002); therefore, a statistically significant difference was found in the IQs of serial killers with regard to organized versus disorganized crime scenes. A moderate effect size of d = 0.42 was calculated (Cohen, 1988).

A second independent sample t-test assessed differences in intelligence level between serial killers whose primary motivations to kill were either financial or for enjoyment. The average IQs of those who killed for financial gain (N = 67) were lower, IQ = 86.84, SD = 19.41. By contrast, the average IQ of those who killed for enjoyment was higher, IQ = 96.5, SD = 26.08. Since the assumption of homogeneity of variance was violated (F = 14.93, p =.000), the degrees of freedom were adjusted from 216 to 167 utilizing the Welch-Satterthwaite method. The end result, t(167) = 3.10, p <.050 (p =.002) indicated a statistically significant difference between the IQs of serial killers and differences in primary motivations (enjoyment, financial) to commit these heinous acts. A moderate effect size of d = 0.43 was calculated (Cohen, 1988).

The next independent sample t-test evaluated IQ differences between serial killers whose enjoyment included rape versus enjoyment without rape. Those who killed for enjoyment with rape (N = 107) had lower IQs, M = 93.074, SD = 23.95, than those who killed for enjoyment without rape (N = 35), M = 104.60, SD = 27.477. The assumption of homogeneity of variance was not violated; hence, equal variances between both groups is assumed with no required adjustment to degrees of freedom. A statistically significant difference was demonstrated between the IQ level of serial killers and motivations to kill for enjoyment with rape versus enjoyment of killing without rape t(140) = -2.38, p <.05 (p =.018). The effect size was found to be d = 0.44, a moderate effect size (Cohen, 1988).

The final independent samples t-test was conducted to further elucidate the association between extremely high and extremely low IQ levels to that of organized versus disorganized crime scene characteristics. An additional filter focused only on serial killers with an IQ of greater than 130 and those with an IQ of less than 70 in order to further segregate specific associations between the most intelligent versus least intelligent serial killers and crime scene characteristics (organized versus disorganized). Results from an independent samples t-test compared differences between those perpetrators who left an organized scene (N = 37, M = 115.19, SD = 38.36) versus a disorganized crime scene (N = 15, M = 79.40, SD = 28.87). Levene's test indicated a violation of homogeneity of variance (F = 8.19, p =.006); therefore, to accommodate this, the degrees of freedom were adjusted from 50 to 34 utilizing the Welch-Satterthwaite method. The result indicated a statistically significant difference of t(34) = 3.67, p <.010 between a filtered group of the most versus least intelligent serial killers and an organized versus disorganized crime scene methodology. Perpetrators with extremely high IQs exhibited a very high frequency of organized crime scene characteristics, while those with very low IQs exhibited a high frequency of disorganized crime scene characteristics. The effect size was found to be d = 1.05, a very strong effect size (Cohen, 1988) based on the magnitude of this difference.


The opportunity to utilize the Radford/FGCU Serial Killer Database (Aamodt et al., 2019), a key strength of this study, resulted in a plethora of statistically significant, unique relationships. Results from both the single sample t-test (Table 2) and the chi-square test for single variance (Table 3) indicated that measures of central tendency (mean, median, mode) and the variation of IQs within the serial killer sample did not model those of the normal population. To pinpoint this variation at both categorical extremes, a binomial test analysis indicated a unique concentration of serial killer IQ levels at both extremely high and extremely low intelligence levels (p<.0005). With an odds ratio of 4.8, perpetrators were almost five times more likely than the normal population to have Very Superior IQs; with an odds ratio of 6.9, perpetrators were almost seven times more likely than the normal population to have Extremely Low IQs (see Table 4). It is important to emphasize that IQ scores in this analysis do not represent a random sample of serial killers (Aamodt et al., 2019). Typically, IQ scores are reported either in the media and/or in court documents when the IQ is unusually high or low. Very high scores (e.g., genius IQs) perpetuate a media frenzy exacerbated by a Hollywood Effect (Cantor & Youngs, 2003; DeLisi et al., 2010), while lower IQs of less than 70 preempt execution for criminal acts as a function of intellectual disability (Atkins v. Virginia, 2002).

Utilization of The Radford/FGCU Serial Killer Database, a large body of information, provided a unique opportunity to quantitatively assess key relationships among intelligence level, methodology, and motivations to serially kill not found in prior research. Analyses from two independent t-tests relative to associations between serial killer IQs and crime scene methodology (organized vs. disorganized) were statistically significant (see Table 5). In fact, a more rigorous analysis (final t-test) segregated extremely high IQs (> 130) and extremely low IQs (<70) to more clearly elucidate specific associations between the most intelligent versus least intelligent serial killers and specific crime scene characteristics. An extremely robust outcome (p <.001, d = 1.05) validated the very strong likelihood that serial killers with extremely high IQs exhibit organized crime scene characteristics, while those with extremely low IQs demonstrate disorganized crime scene characteristics. Therefore, with regard to serial killers, intelligence level (operationally defined as IQ) is an effective predictor of crime scene methodology, and vice versa.

The key discoveries that serial killers who kill for financial purposes have lower IQs than those who kill for enjoyment (p =.002, d = 0.43) (see Table 5) and that perpetrators who enjoy killing but do not rape have higher IQs than those who enjoy and rape (p =.002, d = 0.44), enhances our knowledge of key motivators to serially kill.

Research Implications

Elucidating statistically significant relationships among intelligence, crime scene methodology, and primary motivations to serially kill provides greater clarity relative to myths, preconceptions, and illusions about serial killers. Additionally, the preponderance of IQs within the categories of Extremely Low and Very Superior provided an opportunity to evaluate key differences between these subgroups. This created a unique opportunity on an exploratory basis to delineate differences in methodology and motivation relative to perpetrators in each category (i.e., serial killers at Extremely Low and Very Superior intelligence levels). In general, the results indicated that serial killers with higher IQs display organized crime scene characteristics and are motivated primarily by enjoyment with or without rape as they commit these heinous acts. By contrast, serial killers with lower IQ levels display disorganized crime scene characteristics and are motivated primarily by financial gain.

The results partially support the existence of intellectually gifted prototypes (e.g., Ted Bundy, Ted Kaczynski) who are extremely intelligent, highly organized, and for the most part, kill primarily for enjoyment versus financial gain. Conversely, the existence of a disorganized, less intelligent subgroup of serial killers who are motivated by financial gain rather than enjoyment demonstrates that high intelligence is not universal among serial killers; empirical results from this study dispel the illusory correlation that all prototypical serial killers are highly intelligent. From an applied perspective, the awareness of different crime scene methodologies (organized, disorganized) aligned with perpetrator's motivations and intelligence level equips law enforcement agencies to identify prospective killers more quickly, apprehend them and, in all likelihood, reduce subsequent murders.

I.Q. information available within the Serial killer database represents a convenience sample of perpetrators with very high or very low I.Q.'s, not representative of serial killer I.Q.'s in general; this limits generalizability from these results to what might constitute a "population" of serial killers. Only a small yet impactful subset of serial killers had both recorded I.Q.'s and designations as either organized or disorganized. Only those who met this criteria (both elements present) were included in this analysis. However, identification of this high impact subset (a key strength of this study), for the first time, allows us to glean statistically significant (versus anecdotal) associations among intelligence level, methodology and motivations to serially kill.

To improve our understanding of serial killers, subsequent research should focus on gender differences and variations within race/ethnicity relative to intelligence levels, methodology, and motivations to serially kill. From an applied perspective, such information enhances early identification and apprehension due to greater awareness of racial and gender differences relative to serial killing. From an applied clinical perspective, future research should also focus on relationships among types of parental abuse/early attachment to the commission of heinous crimes by serial killers. A clearer understanding of aberrations by perpetrators (e.g., torture, mutilation) linked to abuse and attachment enhances awareness of these toxic relationships; subsequent intervention at early developmental stages within destructive, highly dysfunctional families may preempt the development of heinous antisocial acts and the propensity to serially kill (Leary et al., 2017). Lastly, due to the associational focus of this study cause and effect relationships among variables cannot be determined.

As a final note, one might hypothesize that present-day, highly intelligent, organized serial killers resemble criminal psychopaths (including serial killers) who, historically, were observed and archived by Pinel (1801), Rush (1812), Krafft-Ebing (1886), and Cleckley (1941, 1976). The combined traits of intelligence, glibness, egocentricity, and excessive charm that are characteristic of these antisocial individuals (Cleckley, 1941) are embedded within the current domain of destructive narcissism (Leary & Ashman, 2018), a key characteristic among contemporary serial killers (Stone & Brucato, 2018).


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Terence Leary, Larry Southard, & Michael Aamodt

Florida Gulf Coast University

Author info: Correspondence should be sent to: Dr. Terence Leary, Whitaker Hall #260, 10561, FL Gulf Coast University, FGCU Blvd., Ft. Myers, FL 33965
Table 1. Wechsler Intelligence Scale

Classification of      IQ Score      Theoretical
Intelligence                           Norm (%)

Very Superior        130 and above       2.2%
Superior                120-129          6.7%
High Average            110-119         16.1%
Average                 90-109          50.0%
Low Average              80-89          16.1%
Borderline               70-79           6.7%
Extremely Low          Below 70          2.2%

Table 2. One Sample T-Test
                          df       t       d      P-value

IQs of serial killers    302    -4.695    0.27   p < .0001
vs. normal population

Note: The calculated median (85) and mode (76) scores for
serial killer IQs also contrasted with the normal adult
population, which was 100 for both measures (Wechsler, 1997).

Table 3. Chi-Square Test for Single Variance

Variance     df    Population   [chi square]     P-Value

633.805     302       225         850.707      p < .0001

Table 4. Binomial Test Analysis

                        N    Observed    Expected    Cohen's h

Very Superior          46      10.6%       2.2%         .37
(IQ [greater than or
equal to] 130)

Extremely Low          32      15.2%       2.2%         .50
(IQ [less than or
equal to] 70)


Very Superior          p< .0005
(IQ [greater than or
equal to] 130)

Extremely Low          p< .0005
(IQ [less than or
equal to] 70)

Table 5. Independent Samples t-Test Analyses

                                      df     t       d       P -Value

Organized vs. Disorganized            190    3.16    0.42    .002 **
Financial vs. Enjoyment               167    3.10    0.43    .002 **
Enjoyment-Rape vs.                    140    2.38    0.44    .018 *
IQ Level [greater than or equal to]   34     3.67    1.05    .001 **
130 vs. [less than or equal to] 70

* Denotes significance at the .05 level
** Denotes significance at the .01 level
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Author:Leary, Terence; Southard, Larry; Aamodt, Michael
Publication:North American Journal of Psychology
Date:Dec 1, 2019
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