Small initial superiority
I was following links from people who had commented here in the past, and wound up reading an excerpt from the Chronicle of Higher Education on this site :
Meanwhile, academics reflected on research by the late Robert K. Merton (a notable mentor himself) and his students on the accumulation of advantage in science, the idea that a small initial superiority can be multiplied over time. A strong undergraduate degree leads to acceptance in a leading Ph.D. program with superior teachers and facilities, qualifying students for positions that help them compete for grants, which in turn make award-winning research possible. Merton called this process the Matthew Effect, citing Matthew 13:12: "For whosoever hath, to him shall be given, and he shall have more abundance."
Harriet Zuckerman's 1977 book on the scientific elite and American Nobel
laureates had shown how crucial the system of graduate supervision had been; more than half of America's Nobel laureates by the year 1972 had been students, postdoctoral fellows, or junior collaborators with older laureates, and many others had worked with major nonlaureates. For higher education as for business, the lesson seemed to be that mentoring had helped maintain male dominance.
Whew. I'm not sure that a leading PhD program with superior teachers matters that much, but it could apply to being in a better lab with a better mentor, who has more connections and will publicize your work, etc.
As a friend said to me the other day, "It's not who you know, it's who knows you."