The Relationship Between Group Size

and Vigilance Whilst Eating

Dionne Angela Donnelly

PSYC 120 Methods & Statistics I – Practical Report

Abstract

Studies into animal vigilance have found that when group size increases, both number of

scans per minute and average duration of scans decrease. Wirtz and Wawra (1986) found that

this also occurs in humans. Therefore we replicated their study in order to test the reliability

and to re-examine the interpretation of the findings. In a naturalistic observation study, 40

participants were observed whilst eating for 5 minutes. Group sizes varied from 1 to 5

members. Sex, frequency of scans and duration of scans were all noted. It was found that

there was a significant negative correlation between both scans per minute and group size,

and average scan duration and group size. These results support theories of the group size

effect, and replicate those found by Wirtz and Wawra (1986). There are a number or reasons

for why this effect occurs including predation risk and searching for mates.

1

Introduction

Vigilance can be broadly defined as “alertness”, or “watchfulness” (Reber & Reber, 2001).

The study of vigilance in humans stems from research into animal vigilance. With regards to

the latter, it is generally accepted that vigilance is important in order for animals to avoid

predation (there is much evidence which supports this assumption e.g. Blumstein, Daniel &

Evans, 2001; Blanco & Hirsch, 2006). One study into the vigilance behaviour of Greater

Rheas (Rhea Americana) found that as group size increased the proportion of time each

individual bird allocated to scanning the environment for predators decreased (Fernandez,

Capurro & Reboreda, 2003). The vast majority of studies investigating vigilance in animals

have reaped similar results (e.g. Bertram, 1980; Dunbar, 1988, cited in Smith &

Winterhalder, 1992). This negative relationship between vigilance and group size had been

termed the ‘group size effect’. This effect is deemed to be adaptive behaviour because “a

predator attack will be detected by some group member earlier if more individuals are

vigilant” (Pulliam, 1973, cited in Evolutionary Ecology and Human Behaviour, p. 106). This

is clearly beneficial to all members of the group, as it not only decreases the likelihood of

each individual being attacked by a predator, but it also allows more time for other activities,

such as foraging and grooming.

In order to test whether the group size effect also occurs in humans, Wirtz and Wawra (1986)

carried out a study in which they measured the number and duration of scans made whilst

people were eating in a student refectory. They defined a scan as being a period which a

“person looked up from eating... Looking directly at another person at the same table was not

counted as scanning behaviour” (Wirtz & Wawra, 1986, pp 283-286). They found that

individual vigilance did decrease as group size increased, and therefore that the group size

effect is an aspect of human vigilance behaviour. Furthermore, they found that males were

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observed to scan the environment more frequently than females. From these findings, Wirtz

and Wawra noted that vigilance may be an ‘evolutionary leftover’, meaning that it is just a

behaviour that has remained with us through evolution despite there being no functional

purpose for it. This seems an unlikely explanation as it seems more logical to believe that

non-adaptive behaviours will disappear during the evolution process. Wirtz and Wawra also

favoured the interpretation that vigilance decreases in larger groups because individuals

perceive an increase in security. Despite appearing to adequately explain the findings, it is

clear that there may be other reasons why the group size effect still occurs in human groups.

As the study by Wirtz and Wawra (1986) is clearly an important study, and due to the fact

there are a number of issues still surrounding the interpretations of their findings, a

replication was carried out in order to test the reliability of their results and to also re-analyse

their interpretation of the data found in light of more recent evidence. There were also a

number of methodological flaws in the original study, such as sample bias, that were

addressed in the replication. The experimental hypothesis for this replication was “as group

size increases, the number of scans per minute will decrease. Also, as group size increases,

the average duration of each scan will decrease.”

3

Method

Participants:

5 participants were observed in St John’s Food court at lunch-time (approx. 12pm).

Participants were a mixture of males and females of various ages and were obtained using an

opportunity sample. From each group size (1-5) one person was chosen at random to be

observed. There were 3 males and 2 females, one assigned to each group size. The data was

then collated with other members of Practical Methods Group 8 in order to bring the number

of participants up to 40.

Design:

This study used a correlation design in order to measure the relationship between scans per

minute and group size, and average scan duration and group size. In order to avoid gender

bias, a similar number of males and females were used. Also, although it was not possible to

record the age of each individual, the researchers tried to use a range of ages to avoid any

further bias.

Materials:

A wall mirror present in the food court was used. A table was pre-prepared by observer M in

order to record the number/duration of scans (see Appendix 1). Two stopwatches were

necessary, one in order to record the 5 minute duration of the observations and the second to

record the duration of each scan.

Procedure:

Total observation time was 30 minutes. Individuals were observed whilst eating in a seating

area in St John’s Food Court, Liverpool at lunch-time (approximately 12pm). A mirror that

was already present was used to observe the individuals with less chance of being detected as

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space was limited. Groups were identified as having between 1 and 5 members and then one

of the members of each group was chosen to be observed whilst eating for a standardised

time of 5 minutes. If the person finished eating, if it became clear the individual had realised

the observer’s presence, or if the number of people in the group changed then the results were

discarded. Individuals were observed from distances between 2 and 10 metres away from the

observer. A scan was recorded every time the individual glanced or looked outside of their

group context i.e. when they looked up from eating but not at another member of their group.

There were three observers (D, M and W). One to record the five minute length of each

observation period, a second to record the length of each scan, and a third to note these times

down. Observers switched roles in order to avoid any potential observer effects. Then, the

total number of scans and the total duration of all scans were recorded in order to calculate

the average number of scans per minute and the average duration of each scan. Once this data

was collected, it was collated with other members of Practical Methods Group 8 to bring the

sample size up to 40 observations. Of these 40 scores, 4 were observations of less than 5

minutes. However these were still used in the final results section to make comparison of data

easier. The data were then analysed by the statistics package SPSS.

5

Results

In order to measure human vigilance whilst eating, both number of scans and the duration of

these scans were recorded and then averaged. A scan was defined as looking up from eating

but not at another member of the group. Histograms (see appendix 3) and a SPSS skewness

test showed that the data was positively skewed. It was therefore necessary to calculate the

medians for the data rather than means and standard deviations (see Table 1).

Table 1: Median values for scans per minute and average scan duration in relation to group size

Group Size Scans per MinuteAverage Scan

DurationMedian Median

1 2 3.88

2 1.6 3.9

3 1.8 3.1

4 1.4 3

5 1.4 2.2

Table 1 shows that as group size increased, both the frequency of scans per minute and

average scan duration generally decreased. The average number of scans per minute were

almost halved between group size 1 and group size 5 (2 and 1.4 respectively). This is also the

case with average scan duration (3.88 and 2.22 respectively). The most scans per minute were

observed in group size 1, with the greatest being 4 scans. The least scans were observed in

group size 5, the fewest being 0.6 scans per minute (see Appendix 2).

Figures 1 and 2 show that both scans per minute and average scan duration are negatively

correlated with group size. Figure 1 clearly shows a relatively weak negative correlation

between scans per minute and increasing group size, as the data is quite widely spread. In

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comparison, Figure 2 shows a stronger negative correlation, as the data points are more

closely compacted around the line of best fit.

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A Spearman’s correlation analysis showed that there was a significant negative relationship

between scans per minute and group size [N=40, rs=-0.32, p<0.01 (one-tailed)], and average

scan duration and group size [N=40, rs=-0.5, p<0.01 (one-tailed)].

Closer analysis of the results in terms of the relationship between gender and group size show

that in group size 1 females scan less frequently than males but for longer intervals. Also,

male scan duration is longer than female scan duration in all group sizes.

Table 2: Average scans per minute and scan duration classified according to gender and group size

Group SizeMeasure Sex 1 2 3 4 5

Scans per Minute (median frequency)

Female 1.3 1.8 1.8 1.6 0.6

Male 2.85 1.5 2.1 1 1.3

Average Scan Duration (median

seconds)

Female 9.35 3 3.1 2.49 1.7

Male 5.27 4.25 4.15 3 2.35

8

Discussion

This study found a significant negative correlation between scans per minute and group size.

There was also a significant negative correlation between average scan duration and group

size. Therefore the experimental hypothesis was accepted. Consequently, this research

provides support for the group size effect as the findings are similar to those found by

Fernandez, Capurro and Reboreda (2003), (see also Pulliam, 1973; Bertram, 1980; Dunbar,

1988). This study also supports the findings of the Wirtz and Wawra (1986) research being

replicated. Despite the findings being similar to those found in previous studies, there are a

number of different interpretations of the findings and therefore a wide range of possibilities

for further study.

Firstly, an additional variable which could be of importance is spatial position within the

group. Blanco and Hirsch (2002), in their study of ring-tailed coatis (Nasua nasua) found that

those animals at the edge of the group were at higher risk of predation and so had higher

vigilance levels (see also: Krause, 1994). Therefore if the group size effect is an ‘evolutionary

leftover’, as Wirtz and Wawra suggest, a further study should be carried out in order to test

whether being in a central position in a group leads to lower vigilance rates than being on the

periphery of a group.

Secondly, Wirtz and Wawra (1986) explained their results in terms of the group size effect

being due to individuals in the group feeling more secure as the number of group members

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increases. Subsequent studies into human vigilance have not seen this to be a likely

explanation and have examined other possibilities. For example, an important study by

Dunbar, Cornah, Daly and Bowyer (2002) looked at the group size effect in humans in terms

of four different hypotheses. These were: predation risk, searching for friends, mate searching

(seeking a sexual partner) and mate guarding (protecting current partner). Rather than the

classic interpretation of predation risk being responsible for the group size effect, they found

that only the mate searching hypothesis received significant support from their results. Such

findings have a number of implications; they indicate that whilst observing the individual, a

note should be made of what they are looking at when they perform a scan, i.e. if it is another

person or just their general surrounding environment. They also indicate that the

demographics of other members of the group should be recorded in order to examine the

differences between vigilance in single-sex and mixed-sex groups (as vigilance has been

found to be significantly higher in single-sex groups, Dunbar et al., 2002).

Thirdly, more recent research has looked into whether individuals coordinate and alternate

their scans with other members of their group. However, this was found not to be the case,

but it has was found that individuals “reacted to increasing group size by increasing the

length of their interscan intervals” (Wawra, 1988, p. 68). This clearly fits in with the findings

of our study as a decrease in the number of scans alongside a decrease in the average duration

of scans leaves longer periods between scans (interscan intervals). However, Wawra’s (1988)

study indicates some of the methodological flaws of both the Wirtz and Wawra (1986) study

and our replication of it.

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The first, and most important of these flaws is sample bias. Although it was attempted to

avoid such a strong bias by not using a sample entirely consisting of students for example, an

opportunity sample was used, which means that it is unlikely a fair representation of the

population was studied. This has implications for generalising the results to a wider

population. Despite this, attempts were made to ensure that a variation of genders and ages

were observed, meaning that it is possible for the results to be generalised to the wider

population.

A second flaw is that although various ages were considered, it was impractical to discover

the exact ages of those observed. Therefore, a possibility for future study would be to look at

the relationship between age and vigilance rates, as there is the possibility that levels of

vigilance could change over time. This also leads to another possible future study, which

could examine whether vigilance rates in humans increase in the presence of children (an

example of this with animals occurs in Blanco & Hirsch, 2006).

In conclusion, it was found that there is a negative correlation between scans per minute and

group size, and average scan duration and group size. This is in accordance with the majority

of previous research. There are several interpretations of the results, as they could be due to

the ‘evolutionary leftover’ of predation risk, or due to individuals searching for mates.

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Group Size

Sex Scans/Duration Total Average

Scans Length Scans per minute

Scan duration

1 Male 1) 3.962) 1.623) 2.884) 0.765) 2.706) 5.31

7) 2.208) 7.83

8 27.26 1.6 3.41

2 Female 1) 1.752) 0.583) 2.344) 0.405) 0.856) 1.17

7) 1.898) 3.339) 5.3510) 1.7111) 1.1212) 1.93

13) 3.0614) 0.4015) 1.3516) 1.8017) 1.8018) 2.07

19) 0.6320) 1.1221) 1.7522) 0.9423) 0.85

23 38.19 4.6 1.66

3 Male 1) 3.282) 3.153) 3.064) 0.995) 2.206) 0.40

7) 0.588) 2.169) 1.3910) 1.2611) 1.6612) 1.93

13) 0.8514) 2.5215) 0.90

15 26.33 3 1.76

4 Male 1) 13.052) 2.883) 1.664) 5.67

4 23.26 0.8 5.81

5 Female 1) 1.212) 2.073) 2.244) 2.475) 1.396) 1.39

7) 2.07 7 12.84 1.4 1.83

Appendix 1 - Raw Scores (D, M, W)

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Group Size SexTotal Obs.

TimeScans/Min

Average scan duration

2 F 3 2.3 3.93 M 2 2.5 5.21 M 2 3.5 4.94 F 4 1.5 3.64 F 5 2 31 F 5 1.6 3.752 M 5 1.4 43 F 5 1.8 3.254 F 5 1.4 3.285 M 5 0.6 3.331 M 5 2.2 20.51 F 5 4 6.52 M 5 2.4 4.52 F 5 2.4 2.453 F 5 1 3.323 F 5 2.2 6.164 M 5 3.2 1.65 M 5 1.6 2.21 M 5 1.8 2.32 F 5 1.4 2.93 F 5 2 3.14 M 5 1 2.25 F 5 0.6 1.71 F 5 0.65 26.772 M 5 1 2.553 F 5 1.8 1.284 F 5 1.6 1.975 M 5 1.6 1.041 F 5 1 12.22 M 5 1.6 5.53 F 5 0.8 1.754 M 5 0.4 35 M 5 1.4 2.52 F 5 1.33 7.54 M 5 1.2 7.493 F 5 1.2 2.483 M 5 1.6 3.091 M 5 4 5.632 F 5 1.8 34 M 5 0.6 3.99

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Appendix 2 - Raw Scores (Group 8)

Appendix 3 - Histograms

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Appendix 4 - Scatterplots Showing Median Values

15

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Appendix 5- Spearman’s Rho Calculations

Scans per Minute and Group Size

Scans Per Minute0.4 0.6 0.6 0.6 0.7 0.8 1.0 1.0 1.0 1.01 3 3 3 5 6 8.5 8.5 8.5 8.5

Rank

Scans Per Minute1.2 1.2 1.3 1.4 1.4 1.4 1.4 1.5 1.6 1.611.5 11.5 13 15.5 15.5 15.5 15.5 18 21.5 21.5

Rank

Scans Per Minute1.6 1.6 1.6 1.6 1.8 1.8 1.8 1.8 2.0 2.021.5 21.5 21.5 21.5 26.5 26.5 26.5 26.5 29.5 29.5

Rank

Scans Per Minute2.2 2.2 2.3 2.4 2.4 2.5 3.2 3.5 4.0 4.031.5 31.5 33 34.5 34.5 36 37 38 39.5 39.5

Rank

Group Size

Group Size1 1 1 1 1 1 1 1 2 2

4.5 4.5 4.5 4.5 4.5 4.5 4.5 4.5 13 13Rank

Group Size2 2 2 2 2 2 2 3 3 313 13 13 13 13 13 13 22 22 22

Rank

Group Size3 3 3 3 3 3 4 4 4 422 22 22 22 22 22 31 31 31 31

Rank

Group Size4 4 4 4 4 5 5 5 5 531 31 31 31 31 38 38 38 38 38

Rank

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Spearman’s Rho Equation

r s =1− 6 ∑ D 2

N 3−N = 1−

8430063960

=−0.32

18

Scans Per Minute

Group SizeScans per

Minute Rank (R1)

Group Size Rank (R2)

D D2

2.3 2 33 13 21 4412.5 3 36 22 14 1963.5 1 38 4.5 33.5 1122.251.5 4 18 31 -13 1692 4 29.5 31 -1.5 2.251.6 1 21.5 4.5 17 2891.4 2 15.5 13 2.5 6.251.8 3 26.5 22 4.5 20.251.4 4 15.5 31 2.5 6.250.6 5 3 38 -35 12252.2 1 31.5 4.5 27 7294 1 39.5 4.5 35 12252.4 2 34.5 13 21.5 462.252.4 2 34.5 13 21.5 462.251 3 8.5 22 -13.5 182.252.2 3 31.5 22 9.5 90.253.2 4 37 31 6 361.6 5 21.5 38 -16.5 272.251.8 1 26.5 4.5 22 4841.4 2 15.5 13 2.5 6.252 3 29.5 22 7.5 56.251 4 8.5 31 -22.5 506.250.6 5 3 38 -35 12250.7 1 5 4.5 0.5 0.251 2 8.5 13 -4.5 20.251.8 3 26.5 22 4.5 20.251.6 4 21.5 31 -9.5 90.251.6 5 21.5 38 -16.5 272.251 1 8.5 4.5 4 161.6 2 21.5 13 8.5 72.250.8 3 6 22 -16 2560.4 4 1 31 -30 9001.4 5 15.5 38 -22.5 506.251.3 2 13 13 0 01.2 4 11.5 31 -19.5 380.251.2 3 11.5 22 -10.5 110.251.6 3 21.5 22 -0.5 0.254 1 39.5 4.5 35 12251.8 2 26.5 13 13.5 182.250.6 4 3 31 -28 784

∑D2 14050

Average Scan Duration and Group Size

Average Scan Duration1.04 1.28 1.60 1.70 1.75 1.97 2.20 2.20 2.30 2.451 2 3 4 5 6 7.5 7.5 9 10Rank

Average Scan Duration2.48 2.50 2.55 2.90 3.00 3.00 3.00 3.09 3.10 3.2511 12 13 14 16 16 16 18 19 20Rank

Average Scan Duration3.28 3.32 3.33 3.60 3.75 3.90 3.99 4.00 4.50 4.9021 22 23 24 25 26 27 28 29 30Rank

Average Scan Duration5.20 5.50 5.63 6.16 6.50 7.49 7.50 12.20 20.50 26.7731 32 33 34 35 36 37 38 39 40Rank

Group Size1 1 1 1 1 1 1 1 2 2

4.5 4.5 4.5 4.5 4.5 4.5 4.5 4.5 13 13Rank

Group Size2 2 2 2 2 2 2 3 3 313 13 13 13 13 13 13 22 22 22

Rank

Group Size3 3 3 3 3 3 4 4 4 422 22 22 22 22 22 31 31 31 31

Rank

Group Size4 4 4 4 4 5 5 5 5 531 31 31 31 31 38 38 38 38 38

Rank

19

Spearman’s Rho Equation

20

Average Scan

DurationGroup Size

Average Scan

Duration Rank (R1)

Group Size Rank (R2)

D D2

3.9 2 26 13 13 1695.2 3 31 22 9 814.9 1 30 4.5 25.5 650.253.6 4 24 31 -7 493 4 16 31 -15 2253.75 1 25 4.5 20.5 420.254 2 28 13 15 2253.25 3 20 22 -2 43.28 4 21 31 -10 1003.33 5 23 38 -15 22520.5 1 39 4.5 34.5 1190.256.5 1 35 4.5 30.5 930.254.5 2 29 13 16 2562.45 2 10 13 -3 93.32 3 22 22 0 06.16 3 34 22 12 1441.6 4 3 31 -28 7842.2 5 7.5 38 -30.5 930.252.3 1 9 4.5 4.5 20.252.9 2 14 13 1 13.1 3 19 22 -3 92.2 4 7.5 31 -23.5 552.251.7 5 4 38 -34 115626.77 1 40 4.5 35.5 1260.252.55 2 13 13 0 01.28 3 2 22 -20 4001.97 4 6 31 -25 6251.04 5 1 38 -37 136912.2 1 38 4.5 33.5 1122.255.5 2 32 13 19 3611.75 3 5 22 -17 2893 4 16 31 -15 2252.5 5 12 38 -26 6767.5 2 37 13 24 5767.49 4 36 31 5 252.48 3 11 22 -11 1213.09 3 18 22 -4 165.63 1 33 4.5 28.5 812.253 2 16 13 3 93.99 4 26 31 -5 25

∑D2 16042.5

r s =1− 6 ∑ D 2

N 3−N = 1−

9625563960

=−0.5

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