Seasonal Variation in Homicide and Assault Across Large U.S. Cities
| Author | David McDowall,Karise M. Curtis |
| Published date | 01 November 2015 |
| Date | 01 November 2015 |
| DOI | http://doi.org/10.1177/1088767914536985 |
| Subject Matter | Articles |
research-article2014
Article
Homicide Studies
2015, Vol. 19(4) 303 –325
Seasonal Variation in
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DOI: 10.1177/1088767914536985
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Across Large U.S. Cities
David McDowall1 and Karise M. Curtis1
Abstract
Although most crimes follow seasonal cycles, homicide is an apparent exception.
The absence of homicide seasonality is surprising given that assault, a closely related
offense, has an obvious annual pattern. Focusing on large U.S. cities, this article
reevaluates seasonality in homicide rates using data with more extensive spatial and
areal dimensions than in previous research. Panel decompositions reveal seasonal
cycles in both homicide and assault rates. Seasonality stands out more clearly in
assault, however, and the patterns differ somewhat in their details. The findings
support the idea that assault and homicide have similar seasonal fluctuations, but they
also suggest that the crimes are more distinct than criminologists often believe.
Keywords
seasonality, trends, homicide, assault, correlates
Seasonal patterns are helpful in understanding the mechanisms that underlie variations
in crime rates, and they have been the subject of a long history of criminological
research. A puzzling issue that arises from this work is the apparent absence of season-
ality in homicide. Some studies do find that homicide exhibits a seasonal cycle, but
these are exceptions to the more usual conclusion that the crime lacks any meaningful
annual structure. The negative results are especially notable given that the closely
related offense of aggravated assault clearly shows the presence of strong and well-
defined seasonality.
The current article reconsiders whether homicide possesses a seasonal pattern, and
if it does, whether this pattern is the same as the one for assault. The article evaluates
1University at Albany, State University of New York, USA
Corresponding Author:
David McDowall, School of Criminal Justice, University at Albany, State University of New York, 135
Western Avenue, Albany, NY 12222, USA.
Email: dmcdowall@albany.edu
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Homicide Studies 19(4)
the seasonal cycles in both crimes through three related analyses. The first of these
examines seasonality in homicide and assault with panel data that include more areas
and a much longer time dimension than previous efforts have used. Short time series
and small geographical samples could have influenced findings about crime seasonal-
ity overall, and about homicide seasonality in particular. If so, seasonal fluctuations
should reveal themselves with greater clarity in a more extensive set of data.
The second analysis uses the large temporal and areal dimensions to assess the
stability of seasonal patterns across time and space. If seasonality is not temporally
and spatially constant, studies could reach different conclusions depending on the
composition of the samples that they analyze. Much of the divergence in the findings
of past research could then be due to variations in the choice of areal units and time
intervals. This would especially be true if homicide seasonality is more sensitive to
sample features than is seasonality in assault.
The third analysis considers if monthly temperature differences mediate the influ-
ence of homicide and assault seasonality. Major criminological theories suggest that
temperatures will carry the impact of seasonal changes, so that the strength of annual
fluctuations varies with the extremity of environmental differences. Here again, find-
ings about seasonality could depend on the geographical units under consideration.
In the following sections, the article first discusses past efforts to study seasonal
variations in violent crime rates and reviews the mostly negative conclusions that they
have reached about homicide. It then describes the current study’s data and methods
and presents the findings of the three analyses. The results show that homicide rates
possess a seasonal structure that is broadly similar to the pattern in assault. Homicide
seasonality is weaker than is seasonality in assault, however, and the annual cycles for
the offenses also diverge in other key respects. The final section considers the implica-
tions of this outcome for research on seasonality and for the relationship between
homicide and assault more generally.
Findings on Seasonality in Homicide and Assault
Systematic investigations of seasonality began early in criminology, and since Aldophe
Quetelet’s (1842/1969) pioneering efforts, studies that examine seasonal crime fluc-
tuations have appeared at a steady pace. A usual finding from this research is that seri-
ous assault follows a seasonal cycle that peaks during the summer. This result so
consistently appears across different time periods and geographical units that Rock,
Judd, and Hallmayer (2008) suggest it is true universally. A few examples of analyses
that find such a pattern are Block (1984); Cohen (1941); Cohn and Rotton (1997);
Deutsch (1978); Dodge (1988); Harries and Stadler (1989); Hird and Ruparel (2007);
McDowall, Loftin, and Pate (2012); Michael and Zumpe (1983); and Rock et al.
(2008).
In contrast, existing research has produced mixed but largely negative results on
seasonal fluctuations in homicide. Some past studies have been successful in detecting
seasonality in homicide data, but by far the more frequent conclusion is that the crime
contains no clear annual pattern. Studies that report seasonal cycles in homicide
McDowall and Curtis
305
include, among others, Hakko (2000); McDowall et al. (2012); Rock, Greenberg, and
Hallmayer (2003); Tennenbaum and Fink (1994); and Warren, Smith, and Tyler
(1983). Some of the more numerous studies that find no homicide seasonality are
Abel, Strasburger, and Zeidenberg (1985); Block (1984, 1987); Brearley (1932);
Cheatwood (1988); Deutsch (1978); Harries (1989); Landau and Fridman (1993);
Michael and Zumpe (1983); Rock et al. (2008); Schmid (1926); and Wolfgang (1958).
Comprehensive reviews by Block (1984), Cheatwood (1988), and Rock et al. (2008)
all also conclude that the research literature offers little support for the existence of
seasonality in homicide rates.
Rock et al. (2008) point out that most studies have lacked data from identical
areas and time periods, and that they have therefore not examined assault and homi-
cide side-by-side. This mismatch complicates attempts to compare the two crimes,
since it opens the possibility that the differences in findings may be due to irrelevant
features of the samples. The few studies that used the same settings and analysis
periods have nevertheless yielded results much like those from other research. Rock
et al. (2008) note, for example, that only a handful of Block’s (1984) analyses of
identical units showed seasonality in both offenses. Studies of comparable areas by
Michael and Zumpe (1983) also found seasonal variation only in assault, and
research in England and Wales yielded the same result (Rock et al., 2008). In studies
of city-level data, Deutsch (1978) and Harries (1989) similarly discovered evidence
of assault seasonality only.
One of the exceptions to finding seasonal variation in assault but not in homicide
was McDowall et al. (2012), who observed annual cycles in both crimes. This out-
come is notable because McDowall et al. used city-level panel data that covered the
largest set of areas and the longest time span of the existing research. Like their study,
the current analysis uses data from a panel of cities. McDowall et al. focused on gen-
eral characteristics that operated across many crimes, however, and they gave little
attention to individual offenses. The present study more closely examines similarities
and differences specific to seasonality in homicide and assault rates. It also uses a
longer time series than did McDowall et al., and it undertakes analyses specifically
tailored to the nature of the two offenses.
Explaining the Differences Between Homicide and
Assault Seasonality
Given the similar circumstances surrounding assault and homicide, one might expect
both crimes to have the same seasonal characteristics. The apparent absence of homi-
cide seasonality has accordingly been a matter of concern to researchers, and they
have proposed a variety of explanations to account for it. A notable example is Block
(1984), who argues that neither assault nor homicide is in fact truly seasonal. Assaults
are more likely to occur outdoors during the summer, however, increasing the chances
that they will come to police attention then. Most studies use police-compiled Uniform
Crime Report (UCR) data, and the summer assault peaks may therefore exist only as
the result of a recording artifact.
306
Homicide Studies 19(4)
Later analyses of victimization data did not strongly support Block’s hypothesis, at
least for serious violence. Carbone-Lopez and Lauritsen’s (2013) study of the National
Crime Victimization Survey (NCVS) found that a large seasonal cycle in aggravated
assault remained after controlling for reporting behavior. Seasonality in less serious
simple assaults did have a reporting component, and seasonal fluctuations in both
aggravated and simple assaults differed across age groups. Overall, however, Carbone-
Lopez and Lauritsen’s results suggest that crime reporting does not...
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