Notes
These notes give the sources behind the claims in each chapter, and say where I'm giving my own interpretation. The book isn't medical advice. If low mood, anxiety or difficulty with everyday tasks is affecting your relationships, your GP is the person to ask. The examples throughout, the cake for the seventieth, the eleven open messages, the friend in Bristol, the friend with the newborn, Dan's fortieth, are invented to illustrate and aren't drawn from real people.
Studies were checked on 26 September 2026 by reading their published abstracts. None was read in full, and none carried a retraction notice at that date. One carries a correction, noted under chapter 12. Several of the studies are about strangers, job candidates or students rather than established friendships, and I've said each time where I'm extending a result beyond what it tested.
Chapter 1. No sources. The terms are the book's own.
Chapter 3. Steel, "The nature of procrastination: a meta-analytic and theoretical review of quintessential self-regulatory failure", Psychological Bulletin 133, 2007, pages 65 to 94. A meta-analysis of 691 correlations. Task aversiveness and task delay are the reviewer's terms. The claim that the proper reply is a standard and not a task is mine. The earlier book referred to is It can be done today.
Chapter 4. Kalman and Rafaeli, "Online pauses and silence: chronemic expectancy violations in written computer-mediated communication", Communication Research 38, 2011, pages 54 to 69. Managers evaluated job candidates whose email response times were one day, two weeks or over a month of silence. The extension from hiring to friendship is mine, and the description of how a friend reads nine days is my observation, not a finding.
Chapter 5. No research sources. The earlier book referred to is Ring them first. The account of the turn is mine.
Chapter 6. Liu, Rim, Min and Min, "The surprise of reaching out: appreciated more than we think", Journal of Personality and Social Psychology 124, 2023, pages 754 to 771. A series of preregistered experiments. The abstract gives no sample sizes and none are printed. The claim that the surcharge is the sender's is mine.
Chapter 7. Aknin and Sandstrom, "People are surprisingly hesitant to reach out to old friends", Communications Psychology 2, 2024, article 34, as read in full for Ring them first, where the notes give the figures. The account of what a reply does is mine.
Chapter 8. Roberts and Dunbar, "Communication in social networks: effects of kinship, network size, and emotional closeness", Personal Relationships 18, 2011, pages 439 to 452. 251 women. Roberts and Dunbar, "Managing relationship decay: network, gender, and contextual effects", Human Nature 26, 2015, pages 426 to 450. Whole active networks over 18 months. The abstract gives no sample size. Oswald and Clark, "Best friends forever?: high school best friendships and the transition to college", Personal Relationships 10, 2003, pages 187 to 196. The garden is mine.
Chapter 9. No research sources. The sequence of two silences is my description.
Chapter 10. Hall, "How many hours does it take to make a friend?", Journal of Social and Personal Relationships 36, 2019, pages 1278 to 1296, as verified for Ring them first. The widely quoted hour thresholds aren't in the abstract and aren't printed. Dunbar, "The anatomy of friendship", Trends in Cognitive Sciences 22, 2018, pages 32 to 51, a review. The claim that the delay saves no hours is mine.
Chapter 11. Kumar and Epley, "It's surprisingly nice to hear you: misunderstanding the impact of communication media can lead to suboptimal choices of how to connect with others", Journal of Experimental Psychology: General 150, 2021, pages 595 to 607, as verified for Ring them first. The NHS page on five steps to mental wellbeing, at nhs.uk, read on 26 September 2026. The earlier book referred to is The phone can wait. The door and the room are mine.
Chapter 12. Hales, Dvir, Wesselmann, Kruger and Finkenauer, "Cell phone-induced ostracism threatens fundamental needs", Journal of Social Psychology 158, 2018, pages 460 to 473. Two studies using recalled conversations. A correction was published in the same journal in 2020, volume 160, pages 264 to 266, which I haven't read. The extension from a phone in a conversation to an unanswered message is mine. The earlier book referred to is Look up.
Chapter 13. No research sources. The whole mechanism is my description, drawing on the chapters it refers to. The tap is mine.
Chapter 14. No research sources. The earlier book referred to is You could have said no. The claim about how invitations get allocated is my observation.
Chapter 15. Liu and colleagues, as above, applied to a situation they didn't test, which the chapter says.
Chapter 16. Epley, Kardas, Zhao, Atir and Schroeder, "Undersociality: miscalibrated social cognition can inhibit social connection", Trends in Cognitive Sciences 26, 2022, pages 406 to 418. A review. Differential construal, uncertain responsiveness and asymmetric learning are the authors' terms, put into plain words in the chapter. Liu and colleagues, as above.
Chapter 17. Dunbar 2018, as above, for the opening claim, which is a reviewer's summary rather than a finding and is treated as such. Zhaoyang, Sliwinski, Martire and Smyth, "Age differences in adults' daily social interactions: an ecological momentary assessment study", Psychology and Aging 33, 2018, pages 607 to 618. 173 adults aged 20 to 79, five reports a day for a week. The four costs are my observations.
Chapter 18. Hall 2019, as above. Vergara Hidd, López, Centellegher, Roberts, Lepri and Dunbar, "The stability of transient relationships", Scientific Reports 13, 2023, article 6120. Mobile phone data from the United States, the United Kingdom and Italy. Used for the single claim that many contacts are transient and the lasting ones can be picked out early. The claim that an old friend can't be replaced quickly is my extension.
Chapter 19. No research sources. The earlier book referred to is You've apologised enough.
Chapter 20. No research sources. The earlier books referred to are Say the good thing out loud and Ring them first.
Chapter 22. Gollwitzer and Sheeran, "Implementation intentions and goal achievement: a meta-analysis of effects and processes", Advances in Experimental Social Psychology 38, 2006, pages 69 to 119, for the when-then form. Lally, van Jaarsveld, Potts and Wardle, "How are habits formed: modelling habit formation in the real world", European Journal of Social Psychology 40, 2010, pages 998 to 1009. 96 volunteers, 82 with usable data. The rule hasn't been tested as a package and what I say to expect is my expectation.
Chapter 23. The NHS pages on ADHD in adults and on the symptoms of depression in adults, at nhs.uk, read on 26 September 2026. The earlier book referred to is Go anyway. No claim is made about the effectiveness of any service.
Chapter 24. Lally and colleagues, as above. The extension from a missed day to a missed month is mine.
Looked up and not used. Oswald, Clark and Kelly's 2004 paper on friendship maintenance behaviours in the Journal of Social and Clinical Psychology had no abstract available to me and isn't cited. Kalman and colleagues' 2006 analysis of response times in online communication likewise. Ledbetter, Griffin and Sparks' 2007 study, which followed pairs of best friends over nineteen years and found that similarity and months of closeness at the start predicted closeness at the end, is about what makes a friendship last rather than about delay, and isn't cited. Kumar and Epley's 2018 study of gratitude letters is used in Thank them now and Say the good thing out loud and isn't needed here. Lieberman and Schroeder's 2020 review of online and offline interaction was read and left out.