The philosopher Derek Parfit has died. The New Yorker published an interesting profile of his work a few years ago.

I’ve spent a lot of time in the last several years working with Dan Bartels on research related to Parfit’s thinking. My own knowledge of Parfit’s work was always a bit secondhand. Dan had read his work closely and saw the potential relevance for how people value outcomes in the future, or to put it in psychological and economics terms, for time discounting.  I had been working on time discounting, and Dan got me interested in how Parfit’s ideas might shed light on the topic.

Much of Parfit’s thinking is laid out in his 1984 book Reasons and Persons. His 1971 paper, “Personal Identity” lays out a shorter case for how he thought about present and future selves.

Parfit is interested in an issue widely debated in philosophy, how personal identity can be defined. He starts with a scenario by David Wiggins, in which a person’s brain is removed, split in two, and transplanted into two new bodies, each of which will have the original person’s character and memories. The question is whether the original person has survived, and if so, whether as one or two people? After discussing this case, and an opposite case in which two people’s brains are merged into one, he introduces a third way of thinking about identity:

“But let us look, finally, at a third kind of being.

In this world there is neither division nor union.  There are a number of everlasting bodies, which gradually change in appearance.  And direct psychological relations, as before, hold only over limited periods of time.

Our beings would have one reason for thinking of themselves as immortal.  The parts of each “line” are psychologically continuous.  But the parts of each “line” are not all psychologically connected.  Direct psychological relations hold only between those parts which are close to each other in time. “

In this paper, Parfit proposed that we rethink personal identity in terms of psychological overlap defined by the degree to which psychologically important features are held in common by the present self and a future or past self, which he calls “psychological connectedness”. He goes on to say:

“On this way of thinking, the word ‘I’ can be used to imply the greatest degree of psychological connectedness. When the connections are reduced, when there has been any marked change of character or style of life, or any marked loss of memory, our imagined beings would say, ‘It was not I who did that, but an earlier self.’ They could then describe in what ways, and to what degree, they are related to this earlier self.

This revised way of thinking would suit not only our ‘immortal’ beings.  It is also the way in which we ourselves could think about our lives. And it is, I suggest, surprisingly natural.”

What starts off as an abstract philosophical discussion suddenly has, if you agree with his point of view, clear implications for how we should think about the past and future when making decisions. Parfit goes on to elaborate on the implications for the norm of self-interest. In his view, if a person has low psychological connectedness with the future self, then it makes sense for that person to care less about that future self, and by extension to make decisions that don’t give much weight to long-term consequences. This is presented in normative terms (and has since been widely debated in philosophy).

Descriptively, psychologists and behavioral economists had long noted that people seemed to give more weight to present than future outcomes in making decisions (like choosing $50 now over $100 in 6 months). This occurs to a degree that is difficult to explain in purely economic terms (i.e. in terms of interest rates and inflation).

My first year at Chicago, I saw Dan present a paper he had been working on with Lance Rips, showing that differences in how people weighted near vs. distant future outcomes against the present could be at least partially explained by how people’s subjective psychological connectedness declines over greater lengths of time.

Dan and I then looked at how people who are (or are made to be) more or less connected to their future selves are more vs. less patient in economic decisions. We also studied how the effect of connectedness on decisions depends on whether the person tends to think about (or is reminded to think about) the future consequences of their decisions. Most recently, Stephanie Chen, Dan and I have investigated  which disruptions to an aspect of a person’s identity would have the most impact on reducing subjective connectedness. Dan and quite a few other people (including Hal Hershfield and Shane Frederick) have also been studying these and related questions in the last few years, and I’ve just finished a review paper summarizing the current state of research on this topic.

In Personal Identity, after discussing the norm of self-interest, Parfit then goes on to discuss possible implications for how people might think about death:

“The second consequence which I shall mention is implied in the first. Egoism, the fear not of near but of distant death, the regret that so much of one’s only life should have gone by — these are not, I think, wholly natural or instinctive. They are all strengthened by the beliefs about personal identity which I have been attacking. If we give up these beliefs, they should be weakened.”

[First in a series, see the next post here].