It is a tremendous generalization to speak of all programs having the same default decision, or all members of an admissions committee in a program having the same default decision, or even to assume the existence of a default decision at all.
In reality, a more likely consideration an admissions committee member might use is, "Does this candidate have similar credentials and experience to successful PhD students we have admitted in the past?" (i.e., no "default" decision.)
However, if we're going to generalize and assume the existence of a default decision:
- For programs in which there are a large number of applicants for relatively few positions, the default decision for any applicant is "reject, unless the applicant is extremely strong."
- For programs in which there are relatively few applicants for a large number of positions, the default decision is "accept, unless there is some evidence this student is unsuitable for the program."
Programs at top-ranked departments, in fields where PhDs are in high demand, and where PhD students are generally fully funded, are more likely to fall in the first category.
Programs at lower-ranked departments, in fields where PhDs are not in high demand, and where students usually fund themselves, are more likely to fall in the second category.
The nationality of the applicant is not generally relevant in the "default" decision, barring exceptional circumstances (e.g., admissions committees in nuclear engineering at U.S. universities may reject Iranian students by default since 2012 because they will be ineligible for visas.)