The girls spent 3.8% more of their time looking at the faces than the boys did (an effect size of 0.17), while the boys spent 11.3% more of their time looking at the mobile than the girls did (an effect size of 0.47).
Another way to describe these results would be to say "Boys on average were slightly more interested in the mobile than in the face (51.9% of their looking time vs. 45.6% of their looking time), whereas girls on average were slightly more interested in the face than the mobile (49.4% of their looking time vs. 40.6% of their looking time). The differences in average looking time between the sexes were between 1/6 and 1/2 of the within-sex standard deviations."
The biggest (proportional) sex difference in the study, actually, was a sex difference in the drop-out rate:
102 neonates (58 female, 44 male) completed testing, drawn from a larger sample of 154 randomly selected neonates on the maternity wards at the Rosie Maternity Hospital, Cambridge. 51 additional subjects did not complete testing due to extended crying, falling asleep, or fussiness, so their data were not used. The mean age of the final sample tested was x = 36.7 hrs (sd = 26.03).
Thus 33 boys (= 77-44) vs. 19 girls (= 77-58) "did not complete testing due to extended crying, falling asleep, or fussiness. This is 74% more boys than girls (33/19 = 1.74). I believe that this pattern is typical of infant research, and it always raises the question of whether it contributes to a bias of some sort, since we don't know whether failure to complete the test is independent of the qualities being tested. One issue that occurred to me is the question of how many of the boy babies might have undergone circumcision before being tested. The rate of infant circumcision in the U.K. is generally fairly low these days, but the authors don't tell us what proportion of the male infants might have been circumcized during the period (average 37 hours) between birth and testing.
Let's note next that Spencer's contrast ("From birth, a girl baby tends to be more interested in looking at colors and textures, like those on the human face, while a boy baby is drawn more to movement, like a whirling mobile") was explicitly not explored in this study, since
The mobile was carefully matched with the face stimulus for 5 factors: (a) Color (‘skin color’). (b) Size and © Shape (a ball was used). (d) Contrast (using facial features pasted onto the ball in a scrambled but symmetrical arrangement, following previous studies (Johnson & Morton, 1991)). (e) Dimensionality (to control for a nose-like structure, a 3cm string was attached to the center of the ball, at the end of which was a smaller ball, also matched for ‘skin color’).
(After seeing this picture, I'm inclined, only half-jokingly, to re-interpret the experiment as showing that "Boy babies are innately somewhat more interested in transdimensional monsters than girl babies are".)
And finally, let's note that
The videotapes were coded by two judges … to calculate the number of seconds the infants looked at each stimulus. A second [third? -myl] observer (independent of the first pair and also blind to the infants’ sex) was trained to use the same coding technique for 20 randomly selected infants to establish reliability. Agreement, measured as the Pearson correlation between observers’ recorded looking times for both conditions, was 0.85.
See above for what inter-rater correlation of 0.85 means…
OK, so how did Leonard Sax interpret results like these as justifying the statement that
"The differences were large: the boys were more than twice as likely to prefer the mobile."
Well, Connellan et al. — perhaps recognizing that the data on looking times are underwhelming — found another way to present the results which emphasizes the sex difference:
For each baby, a difference score was calculated by subtracting the percent of time spent looking at the mobile from the percent of time they spent looking at the face. Each baby was classified as having a preference for (a) the face (difference score of +20 or higher), (b) the mobile (difference score of –20 or less), or © no preference (difference score of between –20 and +20).
This is a reasonable approach to the data, in my opinion, but it should be recognized for what it is, namely an effort to emphasize a difference that does not seem different enough in the more straightforward presentation of looking-time proportions. (And I'll again emphasize that the Right Thing to Do is to publish all the raw data — in this case, the table of face time, mobile time, and "other" time for all 102 subjects.)
Quoting the paper:
Examining the cells that contribute most to the chi-square result suggests that the significant result is due to more of the male babies, and fewer of the female babies, having a preference for the mobile than would be predicted. In other words, male babies tend to prefer the mobile, whereas female babies either have no preference or prefer the real face.
In sum, there are certainly differences in the behavior of the male and female babies in this experiment. The biggest difference was that almost twice as many boys as girls were too fussy or sleepy to be tested. The second-biggest difference was that more of the boy babies were more interested in the "mobile", a surpassingly weird object in which various facial features were pasted on a face-sized ball in a scrambled way, with a second and smaller ball attached by string in a nose-like location.
This experiment certainly seems to tell us something about sex-linked behavioral, perceptual, and (perhaps) cognitive differences in neonates. It may tell us something about sex differences in interest in faces, but maybe not — it seems to tell us more about sex differences in fussiness and sleepiness, and sex differences in interest in weirdly scrambled face-like stimuli. If you wanted to be difficult, you could argue that it really shows that the boy babies have learned more quickly what faces are supposed to look like, and are therefore more concerned to try to sort out what's going on with that strange face-like thing with an eye in its chin and its nose on a string.
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