oerinetoiosantontfantd orlyeveel p

efer toyas girlss (Alex78; Ruplay eet al.,ren byHusto

searchrstandi

Gender-linked toys are typically small replicas of objects linked to the and tumble play (Hassett et al., 2008).

Hormones and Behavior 62 (2012) 500504

Contents lists available at SciVerse ScienceDirect

Hormones an

e ldomestic and nondomestic activities that dene the traditional socialroles of women and men (Crabb and Bielawski, 1994; Ruble et al.,2006). Further, modeling and reinforcement of gender-typical toy playare well documented in both naturalistic (e.g., Fagot, 1978) and experi-mental investigations (Pasterski et al., 2005). However, the hypothesisthat acquired associations between objects and adult gender-linked so-cial roles are necessary for the expression of sex-linked toy preferences

There is very limited research on the relation between activity levelsand sex-linked object preferences in children. One exception is early re-search using observer ratings of children's activity levels dened categor-ically (sitting, standing, walking, running) suggesting that in both sexesmale-typical toys elicit higher activity levels (O'Brien and Huston,1985a). Other research, however, has found no association between cat-egorical ratings of activity level and girls' preferences for masculine toysin children is challengedby research showing sierences in two nonhuman primate species (AlHassett et al., 2008). Further, consistent with thences on animal sex-linked behavior (Breed

Corresponding author at: Texas A&M University, M77843, USA. Fax: +1 979 845 4727.

E-mail address: galexander@tamu.edu (G.M. Alexan

0018-506X/$ see front matter 2012 Elsevier Inc. Alhttp://dx.doi.org/10.1016/j.yhbeh.2012.08.008rences for vehicles, balls,socialization perspective.

gens increase the incentive value of activity and movement (Alexander,2003) through brain mechanisms that also support increased roughWhy boys and girls generally differ in prefedolls and dishes appears obvious froma genderBoys in early childhood generally prtools, and constructionmaterials, whererepresent people and household itemConnor and Serbin, 1977; Fagot, 19well-documented sex differences in toyof life (Connor and Serbin, 1977; Servin1977) and are well-established in child1978; Fagot et al., 1986; O'Brien and1999). Yet, despite a sizable body of reof sex-linked toy preferences, our undes that represent vehicles,generally prefer toys thatander and Hines, 1994;ble et al., 2006). Thesemerge in the second year1999; Smith and Daglish,the age of three (Fagot,n, 1985b; Servin et al.,examining the ontogenyng remains incomplete.

creased preferences for male-typical toys (Berenbaum and Hines, 1992).Biological inuences on preferences for objects viewed as cultural ar-

tifacts (Crabb and Bielawski, 1994) appear incongruous. However, bio-logical factors contribute to higher activity levels in boys compared togirls (Campbell and Eaton, 1999) and features of some boy-preferredtoys (i.e., round shape or wheels) enable propulsion or movement.Higher levels of prenatal androgens in other mammals also increase fre-quencies of rough and tumble play through actions on the amygdala(Meaney, 1988). It may be that male primates show preferences fortoys that support a more active play style (Benenson et al., 1997;BerenbaumandHines, 1992;O'Brien andHuston, 1985a) because andro-Introduction exposed to higher (i.e., male-typical) prenatal androgen levels show in-Early androgens, activity levels and toy ch

Gerianne M. Alexander , Janet SaenzDepartment of Psychology, Texas A&M University, College Station, TX 77843, USA

a b s t r a c ta r t i c l e i n f o

Article history:Received 4 May 2012Revised 20 August 2012Accepted 22 August 2012Available online 30 August 2012

Keywords:Sex differencesDigit ratiosAndrogensToy preferencesActigraphy

The hypothesis that strongmale-typical toys was examtechnology (i.e., actigraphy)gender-neutral toys. Digit ratof age. Therewere no signicduring toy play. In contrast, ccomparisons showed that inwhereas infant boys showemale-typical digit ratios in eatoy preferences in girls. Howfactors other than activity lev

j ourna l homepage: www.milar sex-linked toy pref-exander and Hines, 2002;e known biological inu-love et al., 1999), girls

S 4235, College Station, TX

der).

l rights reserved.ices of children in the second year of life

preferences for active play styles contribute to stronger preferences ford in 47 boys and 37 girls at 19-months of age using ambulatory monitoringmeasure activity levels during contact with male-typical, female-typical, andand salivary testosterone levels were measured earlier in children at 34 monthssex differences in digit ratios, salivary testosterone levels, or overall activity levelsact times showed large sex differences in infants' toy preferences. The within-sexgirls had signicant preferences for female-typical toys over male-typical toys,nly a small preference for male-typical toys over female-typical toys. Moreinfancy predicted higher activity counts during toy play and less female-typicalr, in both sexes, activity levels were unrelated to toy preferences suggesting thatreferences contribute to the early emergence of gender-linked toy preferences.

2012 Elsevier Inc. All rights reserved.

d Behavior

sev ie r .com/ locate /yhbeh(Eisenberg-Berg et al., 1979) or toy-specic sex differences in globalratings of activity levels, such that activity levels during play withmale-typical toyswere higher in boys than in girls but activity levels dur-ing play with female-typical play were higher in girls than in boys (Liss,1981).

To provide a stronger test of the proposal that biological factorspredispose boys' to select toys that support active play, we measuredrelevant hormone markers in early infancy (digit ratios and postnatal

501G.M. Alexander, J. Saenz / Hormones and Behavior 62 (2012) 500504hormone levels) and activity levels during toddlers' toy play. Convergingevidence from research on clinical and nonclinical populations suggeststhat the organizing inuence of prenatal androgens on the developingbrain contributes to the greater expression of male-typical behavior inhumans as in other species (Collaer and Hines, 1995). The sexually di-morphic ratio of the second to fourth digits of the hand (Manning etal., 1998) has been proposed as a marker of prenatal androgen action,such that smaller digit ratios are thought to be determinedbyhigher pre-natal testosterone levels (Breedlove, 2010). However, despite increasinguse in research on human hormone-behavior relations, the meaning ofthe biomarker is unclear (Voracek, 2011). Consistent with previous sug-gestions that variability in prenatal androgen levels is not sufcient to ex-plain the development of sex differences in digit ratios (Wallen, 2009),recent experimental evidence in mice indicates that the second to fourthdigit ratios are determined by relative amounts of prenatal testosteroneand estradiol (Zheng and Cohn, 2011). However, whether these ndingsin mice are generalizable to humans will require additional research.

The behavioral signicance of the transient surge in androgen levelsin early postnatal male development is an emerging area of humanhormone-behavior research. In infant boys, serum testosterone levelsincrease to peak levels around 3 months of age and fall to prepubertalvalues around 6 months of age (Forest et al., 1974). Although levels offree testosterone in infancy are much lower than that in adulthood(de Ronde et al., 2005; Huhtaniemi et al., 1986), the postnatal increasein testosterone levels is critical for the normal development of malegenitalia (Main et al., 2005) and higher postnatal salivary testosteronein boys predicts greatermale-typical visual interests during early infancy(Alexander et al., 2009a). Signicantly, the hypothesis that androgenslevels in the early postnatal period may contribute to the organizationof sex-linked behavior in humans (Alexander and Saenz, 2011;Alexander et al., 2009a) is supported by recent ndings that postnatalurinary testosterone levels are associated with greater male-typicalplay behavior and less female-typical play behavior in girls and boys, re-spectively, at 14-months of age (Lamminmaki et al., 2012).

Valid observation systems measuring young children's activity levelrequire extensive training and parental reports of children's activityhavemodest reliability (Pate et al., 2010). For that reason,weused ambu-latory monitoring technology in later childhood to provide precise mea-sures of motor activity during play with male-typical and female-typicaltoys. Actigraphs, small accelerometers that can be attached to the body,provide direct recordings ofmovements that have been validated againstother physiological measures of energy expenditure in young children(e.g., oxygen consumption) (Pfeiffer et al., 2006). This measure of the cu-mulative intensity and frequency of movement is widely used as an ob-jective measure of physical activity in preschool children (Pate et al.,2010), and applied in research on the biological basis of children's activitylevels (Wood et al., 2007) and the identication of activity relateddysfunctions in childhood, such as sleep disruption following surgery(Kain et al., 2002) and attention decit hyperactivity disorder (Inoue etal., 1998). In this research, we used actigraphy to test three relatedhypothesis: 1. if boy-preferred toys support movement or activity, thenactivity counts in children may be higher during play with boy-preferred toys than during play with girl-preferred toys; 2. if activechildren select toys that support movement or activity, then childrenwith higher activity counts may have stronger preferences for male-typical toys; and, 3. if androgens increase the incentive value of highlevels of activity, then smaller (e.g., more male-typical) digit ratios andhigher postnatal testosterone during early infancy may be associatedwith higher activity counts in children during play.

Method

Participants

Participants were 47 boys (mean age=19.34 months

1.62 months) and 37 girls (mean age=19.95 months2.21 months)and their parents recruited from a longitudinal study of the associationbetween postnatal androgens and the development of gender-linkedbehavior (i.e., play, verbal ability, and aggression). Parents gave in-formed written consent and were offered $10 reimbursement for theirtravel expenses and a small gift (a lab t-shirt) for their son or daughter.

Measures and procedures

Hormone markers at 34 months of ageSalivary levels of testosterone, a measure highly correlated with the

physiologically active portion of total circulating testosterone (Arreggeret al., 2007), were collected at 34 months of age. Saliva (b15 ml) fromeach infant was collected by a sterile DeLee suction catheter and imme-diately stored at80 C. Frozen sampleswere shipped overnight in dryice to Salimetrics (State College, PA), where salivary levels of testoster-one in 41 boys and 30 girls were measured in duplicate using enzymeimmunoassays (assay sensitivity b1 pg/ml). Other children did not pro-vide saliva or did not provide sufcient quantities of saliva for analysis.

The ratio of the second to fourth digits was also measured in infantsas a presumed indicator of prenatal hormone action (Manning et al.,2003; McIntyre, 2006). To measure digit ratios, a digital photo scan ofthe child's right hand was obtained and used to calculate the ratio ofthe lengths of the second and fourth digits (2D:4D). The distance in mil-limeters from the basal crease to the tip of the nger wasmeasuredwithdigital Vernier calipers. Two independent raters coded nger-lengthswith excellent inter-rater reliability (ICC>.85).

Toy preferences and activity levels at 19-months of ageBehavioral measures were collected across two play sessions, each

lasting 8-min. After consent from parents, a light (i.e., 16 g) watch-likeactigraph (Actiwatch, Philips Respironics), was attached to the infant'sankle. The infant was then placed by a parent in the center of a circulararray of toys, selected on the basis of previous research on gender differ-ences in toy play (e.g., Pasterski et al., 2005). The array of toys includedtoys typically preferred by boys (blocks, toy vehicles, toy tools), toys typ-ically preferred by girls (baby doll, cosmetics, tea set), and toys playedwith equally by boys and girls (book, puzzle, stuffed animal). Toyswere arranged so that no two toys from the same toy category wereplaced side by side.

Infants were invited to play with the toys in any way they wished.For the rst play session, the parentwas seated on a chair in the periph-ery of the room and did not interact with the child. For the second playsession, parents were invited to play with the infant in any way theywished. Children and their parents were videotaped and tapes werescored later for toy contact (Session 1 and 2). Parent behavior and infantbehavior elicited by interaction with the parent in Session 2, namelyverbalization and aggressive behavior, are the basis of another report(Saenz and Alexander, in preparation).

Results

Toy preferences

The duration of contact with each toy was coded later by two ratersblind to the study hypothesis using Observer XT (Noldus) with excellentinter-rater reliability (ICC>.90). ANOVA for repeated measures withgender (male, female) as a grouping factor and toy type (male-typical,female-typical, and gender-neutral) and session (child-alone, childparent) as a repeated factor on measures of percent contact timeshowed the expected sex by toy type interaction, F (2, 81)=13.98,Pb .001, such that across both sessions boys interacted more with theset of male-typical toys than did girls (45.7%17.3% vs. 25.9%16.8%,Cohen's d=1.16) (Cohen, 1977), girls interacted more with the set offemale-typical toys than did boys (56.1%22.3% vs. 39.3%17.8%,d=.83), and contact with gender-neutral toys was similar in boys

(14.9%14.7%) and girls (17.9%19.4%) (d=.17). A within-in sex

The results of the model using activity counts as the dependent var-iable was signicant, F (5, 58)=2.91, Pb .05, R2=.20, and showed thatthe simple slope for predicting activity levels from digit ratios was sig-nicant in girls, B=0.63, t=2.82, Pb .007, such that a one-unit in-crease in digit ratios was associated with a 0.63 unit decrease in totalactivity counts. In boys, the negative association between activity ac-counts and digit ratios was smaller, B=0.28, and approached signif-icance, t=1.18, P=.053, but the difference between the slopes for girlsand boys was not statistically signicant, B=0.44, t=1.30, P=.19. Thesimple slope for predicting activity levels from postnatal testosteronecontrolling for digit ratios approached signicance in girls, B=0.35,t=1.92, P=.06, such that a one-unit increase in postnatal testosteronelevels was associated with a 0.35 increase in total activity counts. More-over, the interaction between postnatal testosterone and sex, B=0.37,2.18, Pb .05, indicated that the association between postnatal testos-terone and activity levels was signicantly weaker in boys than in girls.

A similar analysis showed no signicant predictors of children'spreferences for the set of boy-preferred toys. However, the modelusing children's preferences for the set of girl-preferred toys as a de-pendent variable was signicant, F (5, 65)=3.71, Pb .01, R2=.19, andshowed that the simple slope for predicting female-typical toy prefer-ences from digit ratios was signicant in girls, B=.44, t=2.45, Pb .05,such that a one-unit increase in digit ratios was associated with a 0.44unit increase in girls' preferences for the set of female-typical toys. Inboys, the association between digit ratios and preferences for girl-preferred toys was small and non-signicant, B=0.10, and the differ-ence between the two slopes for girls and boys was statistically signi-cant, B= .66, t=2.36, Pb .05. The simple slope for toy preferences

0

50

VEHICLE DOLL

Fig. 1. The proportion of male and female infants' contact time (MeanSEM) with thedoll and vehicles (top graph) and their activity counts (MeanSEM) during interactionswith those objects (bottom graph).

502 G.M. Alexander, J. Saenz / Hormones and Behavior 62 (2012) 500504comparison of contact times with the two categories of gender-linkedtoys showed girls displayed a large preference for female-typical toys,d=1.52, whereas boys showed a smaller preference for male-typicaltoys, d=.36. The three-way interaction between session by sex bytoy-type was not signicant, F (2, 81)=0.52, P=.59, indicating thatthese sex differences in toy preferences were stable across both playsessions.

The pattern of results reported above was replicated in the analysisof preferences for the doll and vehicles, toys selected for further analysisbecause of their clear association to social stimuli (a doll is an objectwith human attributes) and mechanical motion (a vehicle has wheels).Once again, the analyses showed the expected sex differences in contactwith the doll, t (82)=3.12, Pb .01, d=.66, and the vehicle, t (82)=3.07,P=.01, d=.71. Girls showed a large preference for the doll over ve-hicles, t (36)=3.49, Pb .01, d=.94, whereas boys showed a smallerpreference for vehicles over the doll, t (46)=1.95, P=.058, d=.47(Fig. 1, top).

Activity levelsFour boys and four girls did not assent to wearing the actigraph

watch. For the remaining infants (43 boys, 33 girls), activity countswere imported into the behavioral coding software, time lockedwith play data, and activity counts across the entire session and dur-ing all interactions with each of the nine toys were extracted for anal-yses. An analysis of activity accounts in boys and girls across the twosessions showed no overall sex difference, F (1, 74)=.001, P=.97,and no session by sex interaction, F (1, 74)=0.84, P=.36. However,there was a main effect of session, F (1, 74)=43.52, Pb .001, suchthat activity counts were signicantly lower in the parentchild ses-sion compared to the child alone session (5614.912337.54 vs.3905.422299.84, d=.74).

Activity counts and gender-linked toy categories (Hypothesis 1)A pairedsamples test of means showed no signicant differences in

average activity counts during play withmale-typical toys compared tofemale-typical toys, t (76)=.45, P=.66. Further, activity levels werealso similar in both sexes regardless of whether they were interactingwith vehicles (41 boys and 28 girls) or the doll (33 boys and 31 girls)(d=.11) (Fig. 1, bottom).

Intra-individual activity counts and preferences for male-typical toys(Hypothesis 2)

Correlations between the percentage of time playing with male-typical toys and the activity counts during play with male-typical toyswere small and non-signicant for the group of infants, r (76)= .06,for boys alone, r (43)= .16, and girls alone, r (33)=.14. Similarly, cor-relations between the percentage of timeplayingwith the vehicle and ac-tivity counts during play with the vehicle were small and non-signicantfor the group of infants, r (76)=.05, and for boys, r (43)= .01, andgirls, r (33)=.16.

Digit ratios, postnatal androgen levels and behavior during male-typicalplay (Hypothesis 3)

There were no signicant sex differences in digit ratios (0.941.03for boys vs. .933.03 for girls, d=.33) or salivary testosterone levels(39.51 pg/ml12.16 for boys vs. 42.6814.21 for girls, d=.24). The re-lationship between digit ratios, postnatal testosterone and children's be-havior was examined in three separate multiple regression analysesusing as dependent variables children's total number of activity counts,percent time interacting with male-typical toys, and percent timeinteracting with female-typical toys. In view of research suggesting sexdifferences in behavioral responsiveness to early androgens (Alexanderet al., 2009a), we dummy coded sex (female=0,male=1) and comput-ed interaction terms by multiplying dummy code values by digit ratios

and postnatal testosterone for each infant.100

150

200

250

300

350

400

Ave

rage

Act

ivity

Cou

nts

MALE FEMALE

0

0.02

0.04

0.06

0.08

0.1

0.12

0.14

0.16

0.18

0.2

VEHICLE DOLL

Prop

ortio

n of

Pla

ying

Tim

e

MALE FEMALEfrompostnatal testosterone controlling for digit ratioswas not signicant

503G.M. Alexander, J. Saenz / Hormones and Behavior 62 (2012) 500504in girls, B=.26, t=1.58, P=.12, or boys, B=.01. Further, there was nosignicant interaction between postnatal testosterone and sex,B= .18,1.16, P=.25, indicated that the simple slope of toy pref-erences from postnatal testosterone did not differ between boys andgirls.

Discussion

In this longitudinal investigation of activity and toy preferences,smaller (i.e., more male-typical) digit ratios predicted higher activitycounts in infants during toy play. In earlier research, activity levelsmeasured by parental questionnaire were higher in girls exposed pre-natally to higher androgen levels because of Congenital Adrenal Hy-perplasia (CAH) (Pasterski et al., 2007) and activity levels denedcategorically were notably lower in girls exposed prenatally to a pro-gestin with androgen antagonist properties (Ehrhardt et al., 1977). Assuch, the present ndings using an objective, precise and continuousmeasure of activity in typically developing infants during toy playsupport the results of earlier research on older children from clinicalpopulations suggesting prenatal androgens enhance activity levels.

Researchers have long asked How does biology make boys prefercars? (Servin et al., 1999). The primary aim of this research was toprovide an empirical test of the proposal that such toy preferencesare shaped by biologically determined tendencies for higher activitylevels in boys. As summarized above, the ndings from digit ratioswere consistent with biological inuences on activity levels duringtoy play. Unexpectedly, however, activity counts in male and femaleinfants were comparable during play with toys typically preferredby boys and during play with toys typically preferred by girls. Activitycounts were also similar during play with vehicles (toys that clearlysupport movement and propulsion) and a doll (a toy that does not).Moreover, intra-individual activity counts and toy preferences wereunrelated, such that infants with higher activity counts were not morelikely to be infants with stronger preferences for male-typical toys. Insum, these ndings are consistent with the results of much earlier re-search based on categorical ratings of activity levels (Eisenberg-Berget al., 1979) and challenge the longstanding proposal that activitylevel preferences are an important determinant of an infant's choice oftoys.

Activity counts in this research did not replicate the general nd-ing that boys are more active than girls (Eaton and Enns, 1986). Al-though activity monitors placed on the hip, wrist and ankle yielddifferent absolute counts, evidence that different placement sites pro-vide similar measures of activity output (i.e., the counts are highlycorrelated) (Heil et al., 2009) suggests an alternative placement is un-likely to have resulted in signicant sex differences in activity levels.Activity levels in young children are highly variable across social con-text and locations (Pate et al., 2010) and sex differences in children'sactivity levels are largest in familiar, unrestrictive, unstructured set-tings (Eaton and Enns, 1986). Our data replicate ndings that toyplay in a novel, indoor setting elicits sex-linked toy preferences inearly childhood (Lamminmaki et al., 2012). However, the results ofthe precisemeasurement of activity indicate that toy play in a novel, in-door setting does not elicit sex differences in activity levels. The appar-ent disassociation between sex-linked play preferences and activitylevels in this age group is further illustrated by the ndings that, where-as the magnitude of sex differences in toy preferences remained stableacross the two short play sessions in this research, activity levels inboth sexes were reduced in the session with the parent.

If not through inuences on activity levels, then how might biologyinuence children's toy preferences? A report of sex differences in per-cent looking times in neonates for a face compared to amobile (matchedfor characteristics such as color, contrast, and shape) (Connellan et al.,2000) has raised the possibility that sex-linked toy preferences buildon innate preferences in girls for social stimuli and in boys formechanical

motion or other perceptual features characterizing gender-linked toys(Campbell et al., 2000). Yet, in a related study of infants at 12-monthsof age measuring the ratio of looking time directed to social stimuli(human faces verbalizing) compared to mechanical motion (movingvehicles, moving parts of vehicles), female infants showed a clear prefer-ence for social stimuli, but male infants showed a relatively small prefer-ence for mechanical motion (Lutchmaya and Baron-Cohen, 2002). Thus,a male preference for mechanical motion over social stimuli appears lesspronounced at a time when sex differences in toy play begin to emerge.

The relevance of neonatal visual preferences for understandingmale-typical toy preferences is also challenged by research suggestingthat gender-linked toy preferences emerge later in boys than girls. Forexample, in eye-tracking research measuring visual preferences for adoll and a truck in infants at 9-months of age (Alexander et al.,2009b), a preference for a doll was strong in girls and weak in boys. Asmall preference for a doll was also found in three investigations ofboys at 12-months of age (Jadva et al., 2010; Serbin et al., 2001;Servin et al., 1999). In contrast, a small preference for vehicles over adoll was observed in the current research of infants at 19-months ofage and in research of similarly aged infants using a preferential lookingtask (Jadva et al., 2010). In sum, studies of toy preferences during therst two years of life suggest that the strong sex-linked toy preferencesobserved in older boys (Alexander and Hines, 1994) begin between 12-and 18-months of age with a relative decrease in the incentive value ofobjects such as dolls.

The shift in boys' preferences from social stimuli (like dolls) tonon-social stimuli (like vehicles) may occur because well-documentedexperiential mechanisms (social learning, cognitive development) arerequired for the emergence of strong gender-typical toy preferences inboys (Jadva et al., 2010). This possibility may explain why girls withCAH who lack signicant social encouragement of male-typical toy play(Pasterski et al., 2005) generally show preferences for male-typical toysthat are stronger than those of unaffected girls but weaker than thoseof boys (BerenbaumandHines, 1992). The signicant contribution of ex-periential factors to sex-linked toy preferences in boys may also explainwhy, like most previous studies examining hormonal inuences onchildren's toy preferences (Hines, 2011), in this research indicators ofprenatal hormone levels associated with girls' toy preferences wereunrelated to those of boys.

In addition to cognitive and social factors that inuence sex differ-ences in toy preferences, the later emergence of sex-linked toy prefer-ences in boys may also be inuenced by possible sex differences in thematuration of brain systems that support object preferences. However,none of these proposed mechanisms account for evidence of male-typical object preferences in male neonates (Connellan et al., 2000). Itmay be relevant in understanding these apparently transient male-typical object preferences that both sexes display predispositions in therst year of life that are hypothesized to promote adaptive interac-tions with caregivers (Bowlby, 1958), including preferences forfaces (Johnson and Morton, 1991) and for upright rather thaninverted biological motion displays (Simion et al., 2008). These nd-ings suggest an additional explanation for sex differences in theemergence of sex-typed toy preferences: that activation of preferencesfor stimulus features that promote attachment to caregivers in early de-velopmentmay temporarily suppress androgen-dependent preferencesfor mechanical motion or other non-social features of objects in boys.One possible test of this hypothesis would be to investigate whethergirls with CAH also show transient neonatal preferences for mechanicalmotion and show developmental shifts in interest from dolls to trucksbetween 12- and 18-months of age.

In this research, activity levels were unrelated to sex-linked toypreferences assessed in very young children in a brief, structuredplay setting. Whether these ndings generalize to other age groupsand other settings merits further investigation. Further, digit ratios,a sexually dimorphic trait (Manning et al., 1998), did not differ be-tween males and females at 34 months of age. Our ndings are com-

parable to the results of other studies of digit ratios and salivary

504 G.M. Alexander, J. Saenz / Hormones atestosterone levels in infants (Knickmeyer et al., 2011; Lutchmaya etal., 2004), evidence that supports the validity of our measurement.However, future research is required to better understand the ontog-eny of the sex differences in digit ratios and to determine whetherdigit ratios measured at later points in child development show sim-ilar associations to sex-linked behavior. It is also noteworthy that sal-ivary testosterone measured early in the postnatal period did notpredict toddler play behavior in this research, whereas total testoster-one exposure based on urinary testosterone levels assessed monthlyacross the rst six months of age did predict sex-typed play in asmaller number of infants at 14 months of age (Lamminmaki et al.,2012). Clearly, further investigation is required to determine whetherthe differences between these study results occurred becausemeasuresof total testosterone in infants provide a more sensitive measure of thebiological actions of testosterone than salivary testosterone levels, amore reliable assessment of inter-individual differences, or whethertestosterone levels at later points in infant development are more inu-ential for the development of sex-linked play behavior.

Acknowledgments

This work was supported by a National Science Foundation GrantBCS-0618411 (GMA). We thank Dr. Teresa Wilcox and members ofthe Infant Cognition Lab for assistance in the recruitment of toddlersand their families.

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nd Behavior 62 (2012) 500504

Early androgens, activity levels and toy choices of children in the second year of lifeIntroductionMethodParticipantsMeasures and proceduresHormone markers at 34months of ageToy preferences and activity levels at 19-months of age

ResultsToy preferencesActivity levelsActivity counts and gender-linked toy categories (Hypothesis 1)Intra-individual activity counts and preferences for male-typical toys (Hypothesis 2)Digit ratios, postnatal androgen levels and behavior during male-typical play (Hypothesis 3)

DiscussionAcknowledgmentsReferences