Receive and You Want to Give Back, Agree Once and You Agree Again: Reciprocity, Foot-in-the-Door and Dunbar's Number

The invisible rules that move relationships

There are rules between people that we follow without being taught. When we receive, we feel we should give back; once we promise, we feel we should keep our word. These rules protect relationships, but they are also used in sales and persuasion. Authority and social proof are covered in the article on behaviour in context, and the door-in-the-face technique in the article on impressions and persuasion, so read them together.

Reciprocity: receiving makes us want to give back

The sociologist Gouldner (1960) argued that a norm of reciprocity — “if you receive help, you should repay it” — exists in almost every society.

Regan’s (1971) experiment shows it well. During a break, a confederate bought a participant a bottle of Coke, then later asked them to buy raffle tickets. Participants who had received the Coke bought about twice as many tickets as those who hadn’t, regardless of whether they liked the confederate. Cialdini presented this as a core principle of persuasion in Influence (1984).

Check question: if you are asked a favour after receiving a free sample or small kindness, ask yourself, “Would I agree to this without the gift?”

Foot-in-the-door: a small yes leads to a big yes

Freedman and Fraser (1966) first asked residents to put a small sticker saying “Be a safe driver” in their window. Two weeks later, when asked to put a large, ugly sign in their front yard, about 76% of those who had taken the sticker agreed, compared with only about 17% of those who hadn’t received the first request.

One explanation is that people who agree once come to see themselves as “the kind of person who helps with this,” making the next request easier to grant. But Burger’s (1999) review found the effect small on average and highly dependent on conditions: it weakened when the two requests came too close together or when the first request was rewarded.

Check question: if a big request follows a small one, judge by “Would I have agreed if this big request had come first?”

Dunbar’s number: is there a limit to close relationships?

The anthropologist Dunbar (1992) applied the relationship between the proportion of the brain taken up by the neocortex and group size in primates to humans, estimating that people can maintain about 150 stable relationships. A study counting how many people sent Christmas cards also found an average of around 150 (Hill & Dunbar, 2003).

The number has critics, though. Lindenfors, Wartel and Lind (2021) recalculated it using the same approach with newer data and statistics and found the estimated range so wide — from a handful to over 500 — that a single figure of 150 is hard to defend.

Still, the idea that “the time and energy we can give to close relationships is limited” is useful. It fits research suggesting that looking after a few truly close relationships does more for happiness and health than growing your contact list.

Summary at a glance

Principle	Original study	Follow-up and check question
Reciprocity	One Coke roughly doubled raffle purchases	”Would I agree without the gift?”
Foot-in-the-door	Small sticker → 76% agreed to the big sign	Average effect small; depends on conditions
Dunbar’s number	About 150 stable relationships	Estimates too wide to fix one number

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References

• Gouldner, A. W. (1960). The norm of reciprocity: A preliminary statement. American Sociological Review, 25(2), 161–178.
• Regan, D. T. (1971). Effects of a favor and liking on compliance. Journal of Experimental Social Psychology, 7(6), 627–639.
• Cialdini, R. B. (1984). Influence: The Psychology of Persuasion. William Morrow.
• Freedman, J. L., & Fraser, S. C. (1966). Compliance without pressure: The foot-in-the-door technique. Journal of Personality and Social Psychology, 4(2), 195–202.
• Burger, J. M. (1999). The foot-in-the-door compliance procedure: A multiple-process analysis and review. Personality and Social Psychology Review, 3(4), 303–325.
• Dunbar, R. I. M. (1992). Neocortex size as a constraint on group size in primates. Journal of Human Evolution, 22(6), 469–493.
• Hill, R. A., & Dunbar, R. I. M. (2003). Social network size in humans. Human Nature, 14(1), 53–72.
• Lindenfors, P., Wartel, A., & Lind, J. (2021). ‘Dunbar’s number’ deconstructed. Biology Letters, 17(5), 20210158.