Dean Kudzai Tafira Interesting question. I haven't read much about theories of international law. With respect to your question concerning the influence of custom, one could hypothesize that this effect would be small. My rationale would be that in the history of the development of law, international law emerged later, and would more likely be influenced by the structures of law that had already been created. In that view, if custom had an effect, it would operate through other forms of law already in existence. In addition, one could hypothesize that the kind of international law being address would matter, so that, for example, the structure of global economic and power relationships at different points in history would affect the nature of international law, and, perhaps, which nations have the ability to influence the content and structure of international law.

This paper answered your question

https://utrechtjournal.org/articles/10.5334/ujiel.434/

Despite the ability of customs to develop civil laws and speed up the status of trade laws, while their role in criminal law is declining, especially in Latin countries that are governed by the principle that there is no crime or punishment except by a legal text.

Undoubtedly, customs have a great influence on laws, especially criminal ones, as they are the first basis from which codified laws arose. Custom, as is well known, is what people are accustomed to in a country and has become binding on them in general. Especially since many criminal laws in different countries may differ in the policy of criminalization and punishment according to their different customs, for example, you may actually find what is considered a crime in one country without another, such as drinking alcohol, gambling and the like.

Il suffit de lire le jugement de Nuremberg, notamment, les parties relatives au crime contre la paix et aux crimes de guerre.

Very true local values establish criminal behavioral parameters and boundaries.

I guess your question is one regarding time. Over the past 425-years Anglican and Catholic systems have mightily influenced international law. Doctrine of discovery, Habeas corpus, ability to call witnesses and suffer testimony against the accused, and punishments less than death have been relatively adopted by the New World from the Old.

I guess it may be better to refine your question to a specific period, and it will be easier to produce thoughts from a comparative nature.

Indeed such laws normally fits with their custom norms values.And international best practices frowns at such local practices

ed.genital mutilation of the girl child.

For the locals they have their values in such local practices.

But the international community looks at the human rights violations

International norms and customs have influenced International law in general as main source of law. It has also influenced criminal International law enforcement especially in the end of International conflicts and wars [Naplionic wars and exile of losing party ]

Then WWII the trial of Tokyo and Nuremberg where influenced by human rights and international law of war and peace which is mostly norms and customs

It is well known that international custom is one of the main sources on which the International Court of Justice is based when issuing its rulings in accordance with article 38 of its rules of procedure, bearing in mind that jurisdiction to hear international criminal crimes is the prerogative of the International Criminal Court, but this does not prevent the rules of international custom from having an impact on the development of the rules of international criminal law.

Law is a set of legal rules that regulate the life of society