Recently, I was pointed to an article in Newsweek which described a study that was done to test the hypothesis that the Buddhist belief that the self (as we generally think of it, a persisting reality) is an illusion would result in Buddhist monks having less fear of death compared to lay Buddhists, Hindus, and Christians.
This hypothesis was thoroughly falsified. The Tibetan Buddhist monks actually reported more fear of self-annihilation upon dying than any of the other groups. And in the test of selflessness (which should be a result of practicing the Buddhist focus on impermanence), they were actually less selfless than others. This was an interesting day for science, and it's always nice to see a hypothesis falsified, because that's scientific progress.
The researcher quoted in the Newsweek article seemed quite surprised by the results. I worry that this is largely because the researcher doesn't understand Buddhism very well, though I could be very wrong about that. My own grasp of Buddhism is better than the average Westerner (as you can see from my extensive writings on it), but is certainly not complete.
At the very least, you can read in the paper they wrote after the study that the researchers relied on knowledgeable Tibetan Buddhist scholar-monks to calibrate their survey questions and understand the degree to which the answers conformed to standard Tibetan Buddhist teaching. That's good methodology.
I do have some suspicions about the possible causes of the research results with regard to the Tibetan Buddhist monks being less selfless than the lay Buddhists in Tibet and Bhutan. I also have some suspicions with regard to the fact that they appeared to be more afraid of self-annihilation. Regarding the fear of self-annihilation, I think they need to do a comparable study with other Mahayana and Theravada Buddhists (both lay and monastic).
The reason I suggest that they should do more research with other Mahayana and Theravada Buddhists is that the Mahayana tradition generally and the Vajrayana tradition of Tibetan Buddhism specifically has some beliefs that are different from Theravada Buddhism which are relevant to how one would view death.
In Mahayana teachings, there's a strong emphasis on buddha-nature and Buddhahood, and actualizing Buddhahood would cause one to essentially live forever as a bodhisattva. Monks are traditionally considered to be the ones most likely to reach that state (as you can read here), and they would have the most to lose by self-annihilation upon death. On the other hand, lay Buddhists have to be resigned to the high probability of suffering a long time (perhaps millions or billions of years) on another plane of existence, so self-annihilation might not look so bad from their perspective.
Theravada teaching tends to more emphasize the cessation of existence within the cycle of saṃsāra (being reborn over and over again and suffering for all or most of eternity). From that perspective too, self-annihilation could look pretty good.
Another important point with regard to the selfishness of the monks when presented with the life-extending medicine is that traditional Buddhist teaching places monks and care for the basic material needs of monks very high on the moral priorities list because they are the most likely to become enlightened and escape saṃsāra. Therefore, one might have less incentive to extend the lifespan of someone who is very likely to die and be reborn in a naraka and suffer for millions or billions of years before getting another chance to be a monk and gain the opportunity to escape saṃsāra.
That said, there may be a deeper and simpler reason that serious Buddhist practitioners who meditate often would be more attached to their own continued existence. During deep meditation, one can find a tranquility or a bliss which far surpasses the banality of daily life in the quality of experience.
One can also notice that while there is no self in the way that we typically think of it as a persisting psychological reality, there is something which is aware of the contents of the psyche, and that something is what remains with us even after a deep meditation which changes us so dramatically that we can no longer pretend that there is a persisting psychological reality which is the ground of our being.
It is this something which is aware that presumably persists through the endless cycles of death and rebirth known as saṃsāra, through both the terrifying and torturous narakas and the highest heavenly planes. One would guess that Buddhist monastics would be highly cognizant of the fact that this something which persists through life after life, if it were to cease, would mean the cessation of their own being, and their chance at living on as an enlightened bodhisattva.
While none of them would believe that a simple lack of a persisting psychological reality (known popularly as the self) is anything to fear because meditation would make it obvious to them that it is not anything to fear, they might be quite fearful of the final cessation of that something which is aware.
After all, they've developed a closeness with it through meditation that most people never develop. They may have become attached to this something through long familiarity, and it may be wrenching to consider losing it forever, this truly persisting thing without which we would not experience bliss or tranquility (so far as we know).
I'm not saying that any of these beliefs or experiences are necessarily causally related to the greater fear of death or the selfish behavior of the Buddhist monks.
I don't know with certainty why Tibetan Buddhist monks would have a greater fear of death than lay Buddhists or members of other religions in the same geographical area.
But I do think the researchers need to consider the complexity of Buddhist beliefs when thinking about these experiments and what they measure.
Related: What is the role of the Sangha in Buddhism?
By Stephen Shephard - Own work, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=1130661