A recent question on this site ("to suspect" vs "to be suspicious of") asks about the difference between "to suspect" and "to be suspicious of." An even more complicated situation involves when to use suspect as an adjective (as in "suspect reasoning" or "a suspect classification") and when to use suspicious (as in "suspicious thoughts" or "a suspicious detective"). Is there a general rule about when to use one adjective or the other? Are they always interchangeable? A somewhat similar question comes up in an older query titled "that things were suspicious".
In this connection I note Wilson Follett's comment (under transitive/intransitive) in Modern American Usage (1966): "Thus, suspicious should designate the persons harboring a suspicion and suspect the person who is the object of it. From this it follows that one cannot be seen carrying a suitcase in a suspicious manner." Presumably, Follett wants the author to use "in a suspect manner" here.
Follett's analysis is clearly prescriptive and just as clearly ignores centuries of frequent contrary usage; but is there any validity to his prescription as a way for writers to avoid possible ambiguity, as in the case of "suspicious behavior" versus "suspect behavior"?
Best Answer
(1) Statistical analysis
I collected some data on the variation between "a suspicious noun" and "a suspect noun". I collected all instances of these forms from the Corpus of Contemporary American English. I then performed a so-called distinctive collexeme analysis. This will show preference of one noun to occur with one particular adjective, either suspicious or suspect.
The following 12 nouns in my dataset are most strongly associated with suspicious (over suspect):
COLLEX O.CXN1 E.CXN1 O.CXN2 E.CXN2 ASSOC COLL.STR SIGNIF SHARED
----------
1 LOOK 0 9.6 34 24.4 Suspicious 23.16873 ***** N
2 PACKAGE 2 14.8 50 37.2 Suspicious 22.30575 ***** Y
3 MAN 0 6.8 24 17.2 Suspicious 16.25188 **** N
4 EYE 0 6.5 23 16.5 Suspicious 15.56498 **** N
5 DEATH 0 5.1 18 12.9 Suspicious 12.14339 **** N
6 CAR 0 4.3 15 10.7 Suspicious 10.10067 ** N
7 GLANCE 0 4.0 14 10.0 Suspicious 9.42146 ** N
8 LUMP 0 3.4 12 8.6 Suspicious 8.06556 ** N
9 PERSON 2 7.1 23 17.9 Suspicious 6.58107 * Y
10 SPOT 0 2.0 7 5.0 Suspicious 4.69044 * N
11 ACTIVITIES 1 4.3 14 10.7 Suspicious 4.56712 * Y
12 FIRE 1 4.3 14 10.7 Suspicious 4.56712 * Y
In contrast, the twelve nouns below are the ones most strongly associated with suspect (over suspicious):
COLLEX O.CXN1 E.CXN1 O.CXN2 E.CXN2 ASSOC COLL.STR SIGNIF SHARED
----------
1 LIST 20 5.7 0 14.3 Suspect 51.47099 ***** N
2 FOOD 8 2.3 0 5.7 Suspect 20.32793 ***** N
3 CLASSIFICATION 6 1.7 0 4.3 Suspect 15.21411 **** N
4 DESCRIPTION 4 1.1 0 2.9 Suspect 10.12165 ** N
5 INFORMATION 3 0.9 0 2.1 Suspect 7.58337 ** N
6 AREA 7 3.1 4 7.9 Suspect 5.96518 * Y
7 AGENT 2 0.6 0 1.4 Suspect 5.05035 * N
8 AGREEMENT 2 0.6 0 1.4 Suspect 5.05035 * N
9 COMPANY 2 0.6 0 1.4 Suspect 5.05035 * N
10 CREW 2 0.6 0 1.4 Suspect 5.05035 * N
11 DEAL 2 0.6 0 1.4 Suspect 5.05035 * N
12 DOCUMENT 2 0.6 0 1.4 Suspect 5.05035 * N
(2) Analysis
One can now use the items that are most distinctively modified by the adjectives suspect and suspicious to speculate on likely differences between the meaning of the two words.
Keep in mind that the results of such an analysis are not black and white - they do not tell you where one word is right and the other wrong, but where one word is preferred over, more likely than, the other.
Based on the data, I would propose the following two hypotheses:
Hypothesis 1:
Suspicious is preferred to convey that something is wrong with a circumstance obtaining in the first place. It is funny that something has occurred or is occurring at all. The whole situation surrounding a thing seems fishy. The reasons for suspicion are indirect or inferred.
Suspect is preferred to convey that there is something wrong with a particular property or properties of an entity. There is a funny feeling about a specific, tangible aspect of a thing. There is concrete evidence in the thing itself that seems fishy. The reasons for suspicion are direct or self-evident.
Consider some of the top items on the suspicious list:
Now consider some of the top items on the suspect list:
Hypothesis 2:
Animate entities are often described as suspicious, but not really as suspect, to express that they have a skeptical, suspecting attitude. One could say that suspicious is more psychological or mental than suspect.
Consider again the suspicious list: