We followed existing cycle-tracking methods (Gangestad et al. 2002
; Haselton et al. 2007
). Subjects were 69 women from the University of California, Los Angeles (UCLA) campus (mean age=20.30; range=18–39). All the women reported regular menstrual cycles between 21 and 33 days in length and none were using hormonal contraceptives.
After initial screening, the women were scheduled for their next possible session (low- or high-fertility) given their current cycle day. Low-fertility sessions were scheduled 4–10 days prior to the next estimated menstrual onset. On average, low-fertility sessions took place 6.36 days prior to actual menstrual onset (s.d.=3.08). High-fertility sessions were scheduled 15–17 days prior to the next estimated menstrual onset. Using an unmarked urine test (Clearblue), all the women were judged to have a luteinizing hormone (LH) surge between 3 days after and 2 days before their high-fertility session. An LH surge typically precedes ovulation by 24–48 hours (Lynch et al. 2006
); thus all the women were near onset of ovulation during their high-fertility session. Within this window, conception probability peaks just before ovulation (Lynch et al. 2006
). Therefore, we estimated days-to-ovulation by adding 2 to days-to-LH surge (mean=1.49, s.d.=1.48) and included this in analyses.
These 69 women were a subset of 114 recruits. Ineligible women showed no LH surge (n
=15), completed low-fertility sessions outside of the luteal phase (as confirmed by next menstrual onset, n
=4), were scheduled incorrectly (n
=2), missed their menstrual onset date by more than 30 days (n
=2) or failed to complete all sessions (n
=20). Vocal recordings were not obtained for two women due to experimenter error. At the end of the study, the women completed biographical questionnaires. Of women completing the entire study (n
=90), women included in analyses and ineligible women did not differ across pertinent biographical variables, including age, relationship status and self-rated attractiveness (p
>0.20). Retention rates in the study are comparable to previous work using similar methods (e.g. Gangestad et al. 2002
Voices were recorded on a digital (16 bit, 44.1
kHz) recorder (Marantz PMD-660 or M-Audio MicroTrack 24/96) with a cardioid condenser microphone (AKG C535 EB) in a quiet room 15–20
cm from the microphone. The women were instructed before recording what speech to produce exactly. Corrections were requested until the participants produced utterances adequately. Female experimenters ran all sessions. Target speech was extracted using Cool Edit Pro
software (v. 2.1); sound files were re-sampled to 11.025
kHz with a low pass anti-aliasing filter.
Participants produced (i) the sentence, ‘Hi, I'm a student at UCLA,’ (ii) five monopthong vowels (‘eh’ as in bet, ‘ee’ as in beet, ‘ah’ as in bought, ‘oh’ as in boat and ‘oo’ as in boot and (iii) the prolonged (~5
s) monopthong vowel ‘ah’.
Samples were analysed using Praat
, v. 4.6.03 (www.praat.org
0 was measured using Praat's autocorrelation algorithm with a search setting of 100–600
Hz. In the sentence, mean F
s.d. and speech rate (mean syllabic duration) were measured across the entire utterance. For the five monopthongs, F
0 and Df were calculated. Df was calculated as the following: (F
1)/3 with calculations of all formants for each vowel, and values averaged for the final Df value. Overall F
0 was calculated by averaging F
0 values across the five vowels. For the single monopthong vowel, frequency and amplitude perturbation were measured by averaging three jitter (local, relative average perturbation, and 5-point period perturbation quotient) and three shimmer (local, 5-point perturbation quotient and 11-point perturbation quotient) values. Harmonics-to-noise ratio (HNR) was calculated. HNR indexes the degree of acoustic periodicity in dB as the periodic energy divided by total energy. F
0 was measured on the single vowel.