If you have similar effects, it makes sense to look for a common cause.
Take legal trends. If you see seemingly coordinated legal changes in different countries (such as laws on abortion, pornography, homosexual marriage, and so on), it makes sense to infer a common cause. Typically, legal arguments are ostensibly based in particular laws (in the U.S., it might be the constitution, say). Yet, when the same trends are occurring in all sorts of countries, is this really a sufficient explanation? Instead, it seems that the constitution (say) is an ad hoc rationale for the change.
What is the common cause? These are ideas or sentiments which accrue in a legal elite who reach a critical threshold in those societies where the changes occur. This will happen, then, by two distinct mechanisms: ideas or sentiments that flow between certain sorts of elites, or the coming into power of certain sorts of elites who have pre-existing ideas or sentiments.
Take homosexual marriage. It is being legalized in various countries not because of the particularities of the legal traditions in those countries, but because a significant portion of the legal elite have decided that it is a good thing. Once this sentiment occurs, these people then start looking around for ways to change the law, typically using whatever legal arguments are at hand.
The upshot of this is: arguing about these sorts of issues based on the constitution, or what have you, is really fighting a symptom. The cause is the prior ideas or sentiments which the group of elites have. If you want to change the trajectory of jurisprudence in a country, you need to look beyond the ad hoc rationales, towards the education or composition of the relevant elites.