I only have an indirect argument:
As a consequence, it is likely (but not certain) that the US unilaterally exited more treaties than an "average" country, not just in absolute number but also as a proportion of treaties entered.
If someone else can find a direct analysis... by all means, that would be a better answer. Clearly a database exist (with per country data) and was used by Helfer, but it doesn't seem to be public. Barely quantitatively, Helfer's paper only says about the US aggregate behavior:
The unilateralist behavior of the United States provides a salient
example. The United States has recently refrained from ratifying—
or has withdrawn from—numerous multilateral agreements that
are widely ratified by other nations and that it at one time championed.
These treaties include the Kyoto Protocol, the Rome Statute
establishing the International Criminal Court, the Landmines Convention, the Comprehensive Nuclear Test Ban Treaty, the Convention
on Biological Diversity, International Covenant on Economic,
Social and Cultural Rights, the United Nations Convention on the
Law of the Sea, and, most recently, the Framework Convention for
Tobacco Control and the Optional Protocol to the Vienna Convention
on Consular Relations. By remaining outside these treaties
through non-entry or exit, the United States has, according to
many observers, cast doubt on its commitment to multilateral cooperation.
And an important comment on another answer, which might get missed (because there are so many comments under it): regarding the US Constitution Treaty Clause:
The body of law governing U.S. foreign policy recognizes three mechanisms by which the United States enters into binding international obligations. The term "treaty" is used in a more restricted legal sense [in the US] than in international law. U.S. law distinguishes what it calls treaties from congressional-executive agreements and executive agreements. All three classes are considered treaties under international law; they are distinct only from the perspective of internal United States law. Distinctions among the three concern their method of ratification: by two-thirds of the Senate, by normal legislative process, or by the President alone, respectively. The Treaty Clause empowers the President to make or enter into treaties with the "advice and consent" of two-thirds of the Senate. In contrast, normal legislation becomes law after approval by simple majorities in both the Senate and the House of Representatives and the signature of the President.
And from https://en.wikipedia.org/wiki/Executive_agreement
In the United States, executive agreements are binding internationally if they are negotiated and entered into under the president's authority in foreign policy, as commander-in-chief of the armed forces, or from a prior act of Congress. For instance, as commander-in-chief the President negotiates and enters into status of forces agreements (SOFAs), which govern the treatment and disposition of U.S. forces stationed in other nations. The president cannot, however, enter unilaterally into executive agreements on matters that are beyond his constitutional authority. In such instances, an agreement would need to be in the form of a congressional-executive agreement, or a treaty with Senate advice and consent.
The U.S. Supreme Court, in United States v. Pink (1942), held that international executive agreements validly made have the same legal status as treaties and did not require Senate approval.
And Pink has been reaffirmed more recently in Dames & Moore v. Regan:
More recently, in Dames & Moore v. Regan, the Court
relied upon, inter alia, the Pink case to sustain President Carter’s
suspension of claims pending in American courts against Iran as
required by the Hostage Release Agreement of 1981, supra, and,
more directly, by Executive order. In light of Pink, the Court indicated
that ‘‘prior cases * * * have recognized that the President
does have some measure of power to enter into executive agreements
without obtaining the advice and consent of the Senate.’’
So simply relying on the US Constitution notion of "treaty" is misleading when there are two more categories under US law that are also treaties under international law.
And the Congressional source on which that Wikipedia page is mostly based also says
The Constitution is silent on procedures for modifying or terminating
treaties, and agreement has not been reached between the branches on a single proper mode. [...]
Twice in recent years the method of terminating a treaty [in the narrow sense, i.e. one approved by 2/3 of the Senate] has
raised serious controversy within the United States. In 1978, President
Carter terminated the defense treaty with the Republic of
China [Taiwan] without the concurrence of either the Senate or Congress
when he established diplomatic relations with the People’s Republic
of China.
In 1977, the new Panama Canal Treaty terminated
the 1903, 1936, and 1955 treaties with Panama. Although a new
treaty was approved by the Senate, some contended that the termination
of the earlier treaties required an act of Congress, thus including
approval by the House of Representatives as well as the
Senate.
It also talks of controversies of treaty (again in the narrow sense) modification and reinterpretation by the Executive, which are more numerous (a bit too long to quote here).
Also some treaties (in the narrow sense, i.e. 2/3 Senate approved) have an explicit clause that allows the President to terminate them; e.g. KORUS:
If the President does invoke Article 24.5, and no further action is taken, KORUS will terminate 180 days after such notice is given.
And enough scholars think that the President can exit a treaty by himself, regardless of clauses in the treaty:
Whereas it was generally understood throughout
the nineteenth century that the termination of treaties required congressional
involvement, the consensus on this issue disappeared in the early parts of the
twentieth century, and today it is widely (although not uniformly) accepted that presidents have a unilateral power of treaty termination.
This after Goldwater v. Carter (on Taiwan defense exit), Kucinich v. Bush (on ABM exit) etc.
Finally, there's the issue of (extended) provisional application of treaties (in the narrow sense) that have been signed but not ratified, e.g. (quoting from Congressional service again):
An example of a treaty pending in the Senate that has been provisionally
applied by executive agreement is the maritime boundary
agreement between the United States and Cuba, signed December
16, 1977. Originally, an executive agreement, by exchange of
notes on April 27, 1977, had established a modus vivendi on a provisional
maritime boundary to serve during that year while negotiations
were underway. The treaty signed on December 16, 1977,
contained a provision that the parties agree to apply the terms of
the agreement ‘‘provisionally’’ for a period of 2 years from January
1, 1978. According to the Department of State, ‘‘this agreement
constituted an executive agreement contained within the text of the
treaty.’’ The treaty was transmitted to the Senate on January 23,
1979, and debated in the Senate, but final action was not taken.
The treaty is still pending in the committee. [as of 2001] The provisional application was subsequently extended for additional periods, most recently
by an exchange of notes of December 30, 1997 and March
30, 1998.
So such a "treaty" is applied for 20+ years and then denounced (say by a new president) does it really look like nothing happened, no exit from the "treaty" from the viewpoint of the international community (or other party to the treaty)?!
And the middle category of congressional-executive agreements is also exteremely important in practice:
In part because the enumerated powers of Congress and the president have been interpreted broadly, most agreements that are proposed as treaties could also have been proposed as congressional-executive agreements. For that reason, the U.S. government has frequently chosen to use congressional-executive agreements rather than treaties for controversial agreements that are unlikely to gain the required supermajority in the Senate. Examples of contentious proposals addressed in the form of congressional-executive agreements include the 1992 North American Free Trade Agreement (NAFTA) and the agreement whereby the United States became a member of the World Trade Organization (WTO) in 1995.
And yes, these congressional-executive agreements are constitutional too because of Field v. Clark. NAFTA was challeged in Made in the USA Foundation v. United States, but this challenge was stopped in the Eleventh Circuit, which cited Field v. Clark (among other things) in their decision.
The congressional-executive agreements are actually how most of the US international-relations sausage is made nowadays (quoting the Congressional service again)
In the period since 1939 executive agreements [this refers to both congressional-executive and just executive] have comprised more than 90 percent of the international agreements concluded. [...] Most executive agreements are concluded under the authority of a statute [and are called congressional-executive in other places; ...] 88.3 percent of international agreements reached between 1946 and 1972 were based at least partly on statutory authority [meaning they were congressional-executive rather than purely executive].
So you can't just pretend these don't matter just because they're not called treaties in the US law.
So arguing from "first principles" by just quoting the Constitution is a pretty poor way to come to any conclusion on this question because: (i) what is actually a treaty in US vs international law and (ii) termination/exit procedures are unclear in US law (for some categories) and varied in practice.
