The Frequency of Wars*
Mark Harrison**
Department of Economics and CAGE, University of Warwick
Centre for Russian and East European Studies, University of Birmingham
Hoover Institution, Stanford University
Nikolaus Wolf***
Wirtschaftswissenschaftliche Fakultät, Humboldt-Universität
Centre for Economic Policy Research
Abstract
Wars are increasingly frequent, and the trend has been steadily upward since 1870. The
main tradition of Western political and philosophical thought suggests that extensive
economic globalization and democratization over this period should have reduced appetites
for war far below their current level. This view is clearly incomplete: at best, confounding
factors are at work. Here, we explore the capacity to wage war. Most fundamentally, the
growing number of sovereign states has been closely associated with the spread of
democracy and increasing commercial openness, as well as the number of bilateral conflicts.
Trade and democracy are traditionally thought of as goods, both in themselves, and because
they reduce the willingness to go to war, conditional on the national capacity to do so. But
the same factors may also have been increasing the capacity for war, and so its frequency.
Keywords: wars, state capacity, democracy, trade.
JEL codes: H56, N40.
* Earlier versions of this paper were presented to the annual Defense Economics
conference held at the Institute for Defense Analyses of the U.S. Department of Defense,
Washington, DC, on November 17, 2008, the University of Birmingham Centre for Russian
and East European Studies current affairs seminar, February 25, 2009, and the German
Historical Institute conference on War in Transnational and Long Run Perspective, Moscow,
July 2, 2010. We thank Alex Apostolides, Stephen Broadberry, Nick Crafts, Jari Eloranta,
Bishnupriya Gupta, James Harrison, Les Hannah, Alex Klein, Bas van Leeuwen, Anandi Mani,
Philippe Martin, Sharun Mukand, Dennis Novy, Eugenio Proto, Jeremy P. Smith, and the
editor and referees, for advice and comments.
** Mail: Department of Economics, University of Warwick, Coventry CV4 7AL, United
Kingdom. Email: mark.harrison@warwick.ac.uk.
*** Mail: Wirtschaftswissenschaftliche Fakultät, Humboldt-Universität, Spandauer Str. 1,
10178 Berlin, Germany. Email: nikolaus.wolf@wiwi.hu-berlin.de.
First draft: December 1, 2008. This version: March 10, 2011.
The Frequency of Wars: Appendix
Table A1. Time series data, 1870 to 2001
Variable
name
1870
1871
1872
1873
1874
1875
1876
1877
1878
1879
1880
1881
1882
1883
1884
1885
1886
1887
1888
1889
1890
1891
1892
1893
1894
1895
1896
1897
1898
1899
1900
1901
1902
1903
1904
1905
1906
1907
1908
1909
1910
1911
1912
1913
1914
1915
1916
1917
1918
1919
1920
1921
Number
of
countries
# of
countries
47
46
43
43
43
43
44
44
46
46
46
46
45
44
44
44
44
45
45
45
45
45
45
46
46
46
46
46
46
46
46
46
47
48
48
49
48
47
48
49
49
49
48
48
47
47
46
46
54
53
59
60
Pairwise conflicts
Excluding
United
Total
States
FW
FW no US
4
5
5
4
2
4
8
4
8
2
4
4
4
3
4
8
6
6
5
3
4
6
1
5
3
7
6
5
10
4
8
5
8
6
6
7
5
6
9
9
9
19
12
13
27
16
15
18
32
23
20
18
2
5
5
3
2
3
7
4
7
2
3
4
4
3
4
6
6
6
3
1
4
5
1
4
3
6
6
5
9
4
6
5
6
3
4
5
4
6
8
8
8
16
11
12
23
15
11
14
27
21
17
16
Democracy measures
Polity 2
Democracy
-2.32
-2.05
-1.65
-1.65
-1.56
-1.46
-1.60
-1.63
-1.65
-1.53
-1.15
-1.13
-1.13
-1.06
-1.15
-1.15
-1.34
-1.34
-1.21
-1.13
-0.96
-1.00
-0.98
-0.96
-0.89
-0.91
-1.00
-1.00
-0.66
-0.66
-0.68
-0.48
-0.40
-0.44
-0.53
-0.47
-0.20
-0.51
-0.07
-0.02
0.21
0.50
0.56
0.93
0.63
0.68
0.78
1.28
1.54
1.63
1.80
2.05
Executive
constraint
Exconst
Political
competition
Polcomp
Foreign
trade
openness
Openness
3.43
3.62
3.63
3.53
3.62
3.64
3.72
3.73
3.73
3.71
3.72
3.75
3.75
3.74
3.74
3.74
3.66
3.66
3.68
3.70
3.72
3.75
3.77
3.77
3.85
3.85
3.81
3.81
3.85
3.85
3.89
3.96
3.94
3.91
3.81
3.87
3.87
3.77
3.94
4.00
4.02
4.14
4.14
4.27
4.12
4.22
4.28
4.42
4.50
4.53
4.55
4.63
4.58
4.75
4.98
5.02
5.00
5.00
4.76
4.69
4.73
4.78
5.00
4.98
4.98
5.12
5.12
5.12
5.02
5.02
5.08
5.08
5.35
5.37
5.37
5.46
5.38
5.29
5.29
5.29
5.51
5.51
5.42
5.48
5.51
5.55
5.62
5.71
5.87
5.76
5.96
5.90
5.87
5.96
5.96
6.08
5.90
5.96
5.96
6.24
5.98
6.06
6.00
6.20
0.11
0.16
0.18
0.17
0.18
0.16
0.18
0.19
0.20
0.21
0.20
0.21
0.20
0.20
0.20
0.20
0.19
0.19
0.19
0.20
0.16
0.20
0.20
0.19
0.20
0.18
0.21
0.21
0.21
0.20
0.16
0.19
0.19
0.19
0.19
0.20
0.20
0.20
0.20
0.19
0.19
0.20
0.21
0.18
0.17
0.19
0.22
0.23
0.20
0.19
0.18
0.13
2
Variable
name
1922
1923
1924
1925
1926
1927
1928
1929
1930
1931
1932
1933
1934
1935
1936
1937
1938
1939
1940
1941
1942
1943
1944
1945
1946
1947
1948
1949
1950
1951
1952
1953
1954
1955
1956
1957
1958
1959
1960
1961
1962
1963
1964
1965
1966
1967
1968
1969
1970
1971
1972
1973
1974
1975
1976
1977
1978
Number
of
countries
# of
countries
62
62
62
63
63
64
64
64
64
64
65
65
67
67
67
66
66
65
61
54
52
52
55
62
65
67
72
74
74
75
75
77
81
83
86
88
89
88
106
110
117
119
122
125
129
130
134
134
136
142
142
143
145
152
152
153
155
Pairwise conflicts
Excluding
United
Total
States
FW
FW no US
11
11
2
6
8
9
6
4
3
6
7
8
14
11
11
15
13
37
67
33
31
18
14
19
9
5
9
24
21
18
21
23
25
31
29
17
38
41
33
38
37
36
34
41
35
33
23
34
17
27
20
28
26
35
35
37
33
11
11
2
6
7
8
6
3
3
6
6
7
13
11
11
15
13
37
66
29
31
18
14
18
7
5
8
23
19
16
20
19
21
30
24
14
33
39
29
31
30
34
29
37
34
28
20
30
15
25
18
25
25
31
31
35
31
Democracy measures
Polity 2
Democracy
1.88
1.56
1.15
0.94
0.79
0.69
0.43
0.18
0.00
-0.16
-0.13
-0.50
-1.01
-1.00
-1.56
-1.10
-0.96
-1.15
-2.17
-1.74
-1.82
-1.66
-0.86
-0.09
0.09
-0.16
-0.11
-0.38
-0.14
-0.28
-0.14
-0.22
-0.45
-0.39
-0.06
0.05
-0.45
-0.26
-0.74
-0.87
-0.85
-1.17
-1.21
-1.26
-1.35
-1.64
-1.48
-1.78
-1.77
-2.20
-2.55
-2.49
-2.38
-2.37
-2.62
-2.65
-2.19
Executive
constraint
Exconst
Political
competition
Polcomp
Foreign
trade
openness
Openness
4.67
4.57
4.41
4.30
4.25
4.23
4.05
3.98
3.87
3.91
3.91
3.75
3.56
3.54
3.32
3.45
3.46
3.32
3.03
3.15
3.11
3.15
3.44
3.67
3.97
3.82
3.88
3.87
3.94
3.91
4.01
4.01
4.01
3.95
4.01
4.06
3.81
3.87
3.77
3.75
3.81
3.65
3.68
3.63
3.54
3.42
3.42
3.31
3.35
3.25
3.17
3.23
3.24
3.26
3.20
3.18
3.34
6.24
6.12
6.02
5.85
5.87
5.92
5.72
5.65
5.59
5.45
5.46
5.23
4.94
4.99
4.72
4.95
5.02
5.06
4.55
4.60
4.59
4.60
5.22
5.56
5.74
5.68
5.46
5.24
5.28
5.31
5.26
5.19
5.03
5.00
5.12
5.05
4.85
4.90
4.49
4.44
4.42
4.32
4.30
4.18
4.23
4.14
4.20
4.08
4.10
3.85
3.66
3.70
3.75
3.67
3.56
3.54
3.74
0.14
0.15
0.15
0.16
0.15
0.15
0.15
0.14
0.12
0.10
0.08
0.08
0.10
0.10
0.10
0.12
0.10
0.11
0.10
0.09
0.09
0.09
0.09
0.09
0.12
0.15
...
0.13
0.13
0.17
0.15
0.14
0.14
0.16
0.16
0.16
0.13
0.13
0.14
0.14
0.14
0.14
0.15
0.15
0.15
0.15
0.16
0.17
0.18
0.18
0.18
0.21
0.27
0.26
0.27
0.27
0.27
3
Variable
name
1979
1980
1981
1982
1983
1984
1985
1986
1987
1988
1989
1990
1991
1992
1993
1994
1995
1996
1997
1998
1999
2000
2001
Number
of
countries
# of
countries
159
160
162
162
163
164
164
166
166
166
166
169
182
183
187
186
187
187
187
187
187
187
187
Pairwise conflicts
Excluding
United
Total
States
FW
FW no US
35
33
34
39
40
46
47
46
72
46
23
13
21
24
35
42
43
44
44
28
43
32
43
29
31
28
34
28
42
43
42
68
41
21
11
17
22
30
36
41
41
41
23
39
25
39
Democracy measures
Polity 2
Democracy
-1.75
-1.94
-1.95
-1.83
-1.63
-1.70
-1.45
-1.33
-1.31
-1.03
-0.56
0.55
1.24
1.92
2.13
2.42
2.39
2.28
2.26
2.42
2.65
2.89
3.09
Executive
constraint
Exconst
Political
competition
Polcomp
Foreign
trade
openness
Openness
3.47
3.38
3.38
3.44
3.51
3.47
3.52
3.57
3.60
3.67
3.76
4.13
4.38
4.52
4.56
4.64
4.60
4.58
4.54
4.60
4.63
4.70
4.75
3.94
3.85
3.86
3.89
3.98
4.02
4.16
4.20
4.24
4.36
4.64
5.19
5.72
5.97
6.11
6.25
6.25
6.24
6.26
6.43
6.59
6.66
6.76
0.30
0.32
0.33
0.31
0.29
0.27
0.27
0.25
0.26
0.27
0.31
0.32
0.31
0.31
0.30
0.32
0.34
0.35
0.37
0.36
0.37
0.41
0.40
Sources.
FW: The Militarized Inter-State Disputes dataset, version 3.1, at
http://www.correlatesofwar.org, described by Ghosn, Palmer, and Bremer, ‘The MID3 data
set’. Disputes are coded from level (1 no action) through 2 (threat of force), 3 (display of
force), 4 (use of force), and 5 (war). We use all disputes of level 3 (the closing of a border or
the dispatch of ships or troops) and above.
Openness and # of countries: Martin, Mayer, and Thoenig, ‘Make trade not war’.
Democracy: the Polity2 (or net democracy), Exconst (executive constraint), and Polcomp
(political competition) variables from the Polity IV dataset at http://www.systemicpeace.org,
described by Marshall and Jaggers, ‘Political regime characteristics’.
4
Table A2. Unit root tests on FW (the frequency of pairwise conflicts)
Null Hypothesis: FW has a unit root
Exogenous: Constant
Bandwidth: 3 (Newey-West automatic) using Bartlett kernel
Phillips-Perron test statistic
Test critical values:
1% level
5% level
10% level
Adj. t-Stat
Prob.*
-2.915191
-3.481217
-2.883753
-2.578694
0.0463
*MacKinnon (1996) one-sided p-values.
Residual variance (no correction)
HAC corrected variance (Bartlett kernel)
65.63298
54.22658
Phillips-Perron Test Equation
Dependent Variable: D(FW)
Method: Least Squares
Date: 01/17/11 Time: 09:05
Sample (adjusted): 1872 2001
Included observations: 130 after adjustments
Variable
Coefficient
Std. Error
t-Statistic
Prob.
FW_NEW(-1)
C
-0.152393
3.267484
0.047833
1.176782
-3.185957
2.776626
0.0018
0.0063
R-squared
Adjusted R-squared
S.E. of regression
Sum squared resid
Log likelihood
F-statistic
Prob(F-statistic)
0.073473
0.066235
8.164465
8532.287
-456.4271
10.15032
0.001813
Mean dependent var
S.D. dependent var
Akaike info criterion
Schwarz criterion
Hannan-Quinn criter.
Durbin-Watson stat
0.292308
8.449069
7.052725
7.096840
7.070650
2.230413
5
Table A2 (continued).
Null Hypothesis: FW has a unit root
Exogenous: Constant
Lag Length: 0 (Automatic - based on SIC, maxlag=12)
Augmented Dickey-Fuller test statistic
Test critical values:
1% level
5% level
10% level
t-Statistic
Prob.*
-3.185957
-3.481217
-2.883753
-2.578694
0.0231
*MacKinnon (1996) one-sided p-values.
Augmented Dickey-Fuller Test Equation
Dependent Variable: D(FW)
Method: Least Squares
Date: 01/17/11 Time: 09:08
Sample (adjusted): 1872 2001
Included observations: 130 after adjustments
Variable
Coefficient
Std. Error
t-Statistic
Prob.
FW(-1)
C
-0.152393
3.267484
0.047833
1.176782
-3.185957
2.776626
0.0018
0.0063
R-squared
Adjusted R-squared
S.E. of regression
Sum squared resid
Log likelihood
F-statistic
Prob(F-statistic)
Sources. As Table A-1.
0.073473
0.066235
8.164465
8532.287
-456.4271
10.15032
0.001813
Mean dependent var
S.D. dependent var
Akaike info criterion
Schwarz criterion
Hannan-Quinn criter.
Durbin-Watson stat
0.292308
8.449069
7.052725
7.096840
7.070650
2.230413
6
Table A3. The trend in FW (pairwise conflict frequency), 1871-2001
Dep. Var. = FW
Regression Statistics
Multiple R
0.8796
R Square
0.7737
Adjusted R Square
0.7702
Standard Error
0.4444
Observations
131
df
SS
Regression
2
86.45
Residual
128
25.28
Total
130
111.7
Coefficients Standard Error
Intercept
0.6187
0.1233
AR(1)
0.5344
0.0747
time
0.009387
0.001827
Note. Time is annual with t = 0 in 1871.
Sources. As Table A-1.
MS
43.23
0.1975
F
218.9
t Stat
5.017
7.150
5.139
P-value
1.71E-06
5.91E-11
1.01E-06
7
Table A4. The trend in FW no US (pairwise conflict frequency, excluding those involving the
United States), 1871-2001
Dep. Var. = FW no US
Regression Statistics
Multiple R
0.8782
R Square
0.7714
Adjusted R Square
0.7678
Standard Error
0.4547
Observations
131
df
SS
Regression
2
89.310
Residual
128
26.47
Total
130
115.8
Coefficients Standard Error
Intercept
0.5552
0.1152
AR(1)
0.5419
0.07404
time
0.009245
0.001855
Note. Time is annual with t = 0 in 1871.
Sources. As Table A-1.
MS
44.65
0.2068
F
215.9
t Stat
4.820
7.319
4.983
P-value
4E-06
2.44E-11
1.99E-06
8
Table A5. The trend in average percentile rank of countries originating conflicts, 1871-2001,
by GDP
Dep. Var. = GDP%R
Regression Statistics
Multiple R
7.969E-03
R Square
6.351E-05
Adjusted R Square
-2.523E-04
Standard Error
0.2779
Observations
3168
df
SS
Regression
1
0.01553
Residual
3166
244.5
Total
3167
244.5
Coefficients Standard Error
Intercept
0.6699
0.01480
time
7.039E-05
1.570E-04
Note. Time is annual with t = 0 in 1870.
Sources. As Table 1; see the text.
MS
0.01553
0.0772
F
0.2011
t Stat
45.26
0.4484
P-value
0
0.6539
9
Table A6. The trend in average percentile rank of countries originating conflicts, 1871-2001,
by GDP per head
Dep. Var. = GDPC%R
Regression Statistics
Multiple R
0.03353
R Square
0.00112
Adjusted R Square
8.085E-04
Standard Error
0.2889
Observations
3168
df
SS
Regression
1
0.2973
Residual
3166
264.2
Total
3167
264.5
Coefficients Standard Error
Intercept
0.4868
0.01539
t
3.080E-04
1.632E-04
Note. Time is annual with t = 0 in 1870.
Sources. As Table 1; see the text.
MS
0.2973
0.08344
F
3.563
t Stat
31.64
1.887
P-value
3.7E-191
0.05919
10
References
Ghosn, F., G. Palmer, and S. Bremer, ‘The MID3 data set, 1993–2001: procedures, coding
rules, and description.’ Conflict Management and Peace Science, 21 (2004), pp. 133-154.
Marshall, M. G., and K. Jaggers, ‘Political regime characteristics and transitions, 1800-2006’,
Center for Systemic Peace, Polity IV Project at http://www.systemicpeace.org (2007).
Martin, P., T. Mayer, and M. Thoenig, ‘Make trade not war?’ Review of Economic Studies
75:3 (2008), pp. 865-900.