In classic experiments on stress: David C. Glass and Jerome Singer, “Behavioral Consequences of Adaptation to Controllable and Uncontrollable Noise,” Journal of Experimental Social Psychology 7 (1971): 244–57; David C. Glass and Jerome E. Singer, “Experimental Studies of Uncontrollable and Unpredictable Noise,” Representative Research in Social Psychology 4 (1973): 165–83.

When people are in pain, they need a button: Brian R. Little, Me, Myself, and Us: The Science of Personality and the Art of Well-Being (New York: Public Affairs, 2014).

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There are two different emotional responses: C. Daniel Batson, Jim Fultz, and Patricia A. Schoenrade, “Distress and Empathy: Two Qualitatively Distinct Vicarious Emotions with Different Motivational Consequences,” Journal of Personality 55 (1987): 19–39.

“As some friends checked in daily”: Allen Rucker, The Best Seat in the House: How I Woke Up One Tuesday and Was Paralyzed for Life (New York: HarperCollins, 2007).

In one experiment, people were asked: Loran F. Nordgren, Mary-Hunter McDonnell, and George Loewenstein, “What Constitutes Torture? Psychological Impediments to an Objective Evaluation of Enhanced Interrogation Tactics,” Psychological Science 22 (2011): 689–94.

the Platinum Rule: This is a term that has been attributed to many sources. One of the best descriptions comes from Karl Popper’s writing. “The golden rule is a good standard which can perhaps even be improved by doing unto others, wherever possible, as they would be done by”: Karl Popper, The Open Society and Its Enemies, vol. 2 (New York: Routledge, 1945/1966).

“while well meaning, this gesture”: Bruce Feiler, “How to Be a Friend in Deed,” The New York Times, February 6, 2015: www.nytimes.com/2015/02/08/style/how-to-be-a-friend-in-deed.html.

“Some things in life cannot be fixed”: Megan Devine, Refuge in Grief: Emotionally Intelligent Grief Support, accessed on December 14, 2016: www.refugeingrief.com/.

Psychologists put teenage girls under stress: Jessica P. Lougheed, Peter Koval, and Tom Hollenstein, “Sharing the Burden: The Interpersonal Regulation of Emotional Arousal in Mother-Daughter Dyads,” Emotion 16 (2016): 83–93.

the “ring theory”: Susan Silk and Barry Goldman, “How Not to Say the Wrong Thing,” Los Angeles Times, April 7, 2013: http://articles.latimes.com/2013/apr/07/opinion/la-oe-0407-silk-ring-theory-20130407.

the five stages of grief: Elisabeth Kübler-Ross, On Death and Dying (New York: Routledge, 1969).

these are not five stages: Holly G. Prigerson and Paul K. Maciejewski, “Grief and Acceptance as Opposite Sides of the Same Coin: Setting a Research Agenda to Study Peaceful Acceptance of Loss,” The British Journal of Psychiatry 193 (2008): 435–37. See also Margaret Stroebe and Henk Schut, “The Dual Process Model of Coping with Bereavement: Rationale and Description,” Death Studies 23 (1999): 197–224. As social worker Carole Geithner explained to us, stage models “also minimize the individuality and diversity of how people grieve. There are different grieving styles and coping strategies. Grief models create problems when they become prescriptive. More current models emphasize individuality. There is an understandable wish for models because we want reassurance, to know there is an endpoint, a game plan, some predictability, but there is a downside: They are not true to the reality of grief. They provide illusory comfort. Each person’s loss is different.”

As people mature, they focus: Laura L. Carstensen, Derek M. Isaacowitz, and Susan T. Charles, “Taking Time Seriously: A Theory of Socioemotional Selectivity,” American Psychologist 54 (1999): 165–81.

the quality of friendships becomes: Cheryl L. Carmichael, Harry T. Reis, and Paul R. Duberstein, “In Your 20s It’s Quantity, in Your 30s It’s Quality: The Prognostic Value of Social Activity Across 30 Years of Adulthood,” Psychology and Aging 30 (2015): 95–105.

“Footprints in the Sand”: Allegorical poem published in various versions. See, for example, http://www.footprints-inthe-sand.com/index.php?page=Main.php.