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Exploring the validity of a new version of the ‘Inclusion of the Other in the Self’ (IOS) task.

Last registered on February 22, 2023


Trial Information

General Information

Exploring the validity of a new version of the ‘Inclusion of the Other in the Self’ (IOS) task.
Initial registration date
July 14, 2021

Initial registration date is when the trial was registered.

It corresponds to when the registration was submitted to the Registry to be reviewed for publication.

First published
July 14, 2021, 10:15 AM EDT

First published corresponds to when the trial was first made public on the Registry after being reviewed.

Last updated
February 22, 2023, 9:03 AM EST

Last updated is the most recent time when changes to the trial's registration were published.


Primary Investigator

University of Zurich

Other Primary Investigator(s)

PI Affiliation
University of Nottingham
PI Affiliation
University of Nottingham
PI Affiliation
University of Nottingham

Additional Trial Information

Start date
End date
Secondary IDs
Prior work
This trial does not extend or rely on any prior RCTs.
Understanding the strength of social relationships is crucial to explain cooperative and other-regarding behaviour. To this end, researchers have developed a multitude of tools to measure connectedness between individuals. In previous work, Gächter, Starmer and Tufano (2015) showcase that the ‘Inclusion of the Other in the Self’ (IOS) scale is an easy, portable tool that reliably measures relationship closeness. Moreover, in the same work and subsequent research the authors find an even stronger predictive power of the ‘Oneness’ scale, that is a combination of the IOS and We-Scale. However, as this scale requires two distinct items, it lacks the ease that the IOS by itself entails. In this project, we design an extended version of the IOS scale that enables subjects to easily indicate relationship closeness with more nuance whilst maintaining its simplicity. We now validate the tool by exploring whether it captures the strength of relationships as well as the ‘Oneness’ scale and other survey measures.
External Link(s)

Registration Citation

Baader, Malte et al. 2023. "Exploring the validity of a new version of the ‘Inclusion of the Other in the Self’ (IOS) task.." AEA RCT Registry. February 22.
Experimental Details


In our study we are exploring the validity of a new, extended version of the 'IOS' task. In order to validate the method against the original tool we will employ treatment variations along two dimensions: 1) Original or extended IOS task 2) Three different relationship levels.
This allows us to explore the validity relative to the original task as well as investigate the task's properties across tighter and looser relationships
Intervention Start Date
Intervention End Date

Primary Outcomes

Primary Outcomes (end points)
Measurement of relationship closeness using a variety of scales and compare the predictive power of Oneness to the proposed extended version of IOS.
Primary Outcomes (explanation)

Secondary Outcomes

Secondary Outcomes (end points)
Secondary Outcomes (explanation)

Experimental Design

Experimental Design

Before outlining the data collection, we describe the two distinct methodologies compared in this study. In the original IOS scale subjects are provided with seven distinct pairs of circles which overlap to a different degree. The overlap represents the strength of relationship between the subject and an individual in question. After considering all seven pairs of circles, subjects then select the pair that most closely represents their relationship with the other individual. A major upside to this methodology is its ease to understand and administer, however, due to the discrete set-up it limits subject’s ability to answer with more nuance. To add the missing nuance researchers have developed the ‘Oneness’ scale. For this measurement the IOS elicitation is combined with the We-Scale, in which subjects answer ‘… to what extent you would use the term “WE” to characterize you and [the individual in question] ’. Even though this combined measurement has been shown to be highly correlated to other measures of relationship closeness and predictive of team production (Gächter et al, 2019), it lacks the simplicity of the original IOS scale.
Thus, when applying the IOS methodology validated by Gächter et al. (2015) researchers frequently solely employ the IOS scale and forego to also elicit Oneness, thereby, maintaining simplicity but losing precision in their measurement or relationship closeness. To address this, we develop an extended version of the IOS scale that combines the simplicity of implementation with more nuance in the measurement. It follows the same graphical style of the original IOS scale but subjects are asked to move a slider in order to adjust the overlap of pairs of circles which appear in a single screen. In order to reduce noise from the choice, the position of the slider is still discrete with now 11 distinct degrees of overlap. Identically to the original IOS, this tool is easy to administer in computerised experiments and provides the subjects with additional opportunities to report relationship closeness in a more finely grained manner.

Data collection

For the data collection we closely follow the approach taken by Gaechter, Starmer, and Tufano (2005). We ask subjects to answer five distinct scales (IOS (original/extended), We-Scale, RCI, LLS, PAM) intended to measure relationship closeness, one of which is either the original IOS scale or our proposed extended version. We then explore in how far ‘Oneness’ or our proposed measure correlate to the different scales, we thereby replicate Gaechter, Starmer and Tufano (2015) whilst in addition exploring the validity of our new methodology.
Moreover, to examine whether the tools are valid across a variety of relationships we also randomly assign subjects to think of a person of one of three relationship levels: close relationship, friend, or acquaintance. Lastly, we also elicit baseline IOS for a stranger, personality traits and self-reported social preferences as additional controls.

Gächter, S., Starmer, C., & Tufano, F. (2019). The surprising capacity of the company you keep: revealing group cohesion as a powerful factor of team production (No. 2019-16). CeDEx Discussion Paper Series.
Gächter, S., Starmer, C., & Tufano, F. (2015). Measuring the closeness of relationships: a comprehensive evaluation of the 'Inclusion of the Other in the Self' scale. PloS one, 10(6): e0129478.
Experimental Design Details
Randomization Method
We will randomise subjects into different IOS scales (original/ extended) as well as relationship levels within the same session using a random number generator integrated by Qualtrics.
Randomization Unit
Was the treatment clustered?

Experiment Characteristics

Sample size: planned number of clusters
750 subjects
Sample size: planned number of observations
750 subjects
Sample size (or number of clusters) by treatment arms
250 per relationship level, 375 per IOS scale (125 observations of each IOS scale within each relationship level).
Minimum detectable effect size for main outcomes (accounting for sample design and clustering)
The study will be conducted using Prolific and is programmed using Qualtrics. The sample sizes are based on the samples used and effect sizes found by Gaechter, Starmer and Tufano (2015).

Institutional Review Boards (IRBs)

IRB Name
Nottingham School of Economics Research Ethics Committee
IRB Approval Date
IRB Approval Number


Post Trial Information

Study Withdrawal

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Is the intervention completed?
Intervention Completion Date
July 25, 2021, 12:00 +00:00
Data Collection Complete
Data Collection Completion Date
July 25, 2021, 12:00 +00:00
Final Sample Size: Number of Clusters (Unit of Randomization)
751 individuals
Was attrition correlated with treatment status?
Final Sample Size: Total Number of Observations
751 invidviduals
Final Sample Size (or Number of Clusters) by Treatment Arms
around 125 subjects per treatment
Data Publication

Data Publication

Is public data available?

Program Files

Program Files
Reports, Papers & Other Materials

Relevant Paper(s)

Reports & Other Materials