Language and life history: A new perspective on the development ...

Locke & Bogin: <strong>Language</strong> <strong>and</strong> <strong>life</strong> <strong>history</strong>those children who experience it, but the rate of growthdeclines even in children without a detectable spurt. Thecause of this decrease in growth rate is unknown, butJanson <strong>and</strong> van Schaik (1993) have proposed severalbenefits. The first is that it provides additional time forthe brain growth <strong>and</strong> learning required for reproductivesuccess in various species of social mammals. Social carnivores,elephants, <strong>and</strong> primates must all learn how to livewithin the social hierarchy of the group. Juvenility alsooffers opportunities to learn complex feeding skills suchas how to hunt, how to open fruits or seeds with protectivecoverings, <strong>and</strong> where <strong>and</strong> when to find food. Reproductiveskills must also be learned, including ways to compete formates <strong>and</strong> care for offspring. Joffe (1997) has reportedpositive correlations between the length of juvenility <strong>and</strong>both group size <strong>and</strong> the volume of social areas of the brain.Janson <strong>and</strong> van Schaik (1993) also proposed that slowergrowth reduces feeding competition <strong>and</strong> maintains asmaller body, which is less threatening to adults. Growingup in a social group that is composed of individuals of differentages <strong>and</strong> social status can be difficult. This is especiallytrue for juveniles who must compete with older, moreexperienced, individuals for food, space, <strong>and</strong> otherresources. Fortunately, their relatively small <strong>and</strong> slowlygrowing bodies require less food than adult bodies, <strong>and</strong>may also afford them some protection from adult competition<strong>and</strong> attack as they present a non-threateningjuvenile morphology.In humans, the decline of growth rate bottoms out atabout age 10 years in girls, two years before it does so inmales, <strong>and</strong> this marks the end of the juvenile stage. Asfor the function of juvenility, it seems reasonable tosuppose that in humans, like the other primates, thisstage offers opportunities to prepare for the social complexitiesof adolescence, as well as adulthood. Wepropose that juvenility also provides the young withchances to learn <strong>and</strong> perfect skills associated with thesocial, pragmatic, <strong>and</strong> performative uses of language.2.6. Juvenile languageCoates (1986) has pointed out that, for all the advancesoccurring in childhood, “a knowledge of grammar, phonology<strong>and</strong> [the] lexicon is not enough – it does not make thechild competent; children need to master not only theformal rules of language, but also rules for the appropriateuse of language” (p. 121). To be sure, juvenility accommodatesadditional syntactic advances (cf. Nippold 1998),with some English forms continuing to emerge in earlyadulthood (Marchman et al. 1991), but many of the <strong>new</strong>developments affect performance. These include anincrease in speech-breathing capacity (Engstrom et al.1956; Hoit et al. 1990) <strong>and</strong> further increases in fluency(Starkweather 1987). Prior to the adolescent “voicebreak,” there are anatomical changes that affect the ratioof the fundamental frequency to the ratio of higherformant frequencies (Sachs et al. 1973; also see Austin &Leeper 1975; Vuorenkoski et al. 1978).There are changes, too, that occur beyond the sentencelevel, in the quality of extended discourse <strong>and</strong> narratives(Bamberg 1987; Burleson 1982; Karmiloff-Smith 1985).These pragmatic advances facilitate a variety of sociallyrelevant activities, from gossip to storytelling, <strong>and</strong> contributeto successful competition <strong>and</strong> courtship in the runup to sexual maturity. In Turkey, boys engage in verbalduels – ritualistic insults <strong>and</strong> replies that require “skill inremembering <strong>and</strong> selecting appropriate retorts to provocativeinsults” (Dundes et al. 1970, p. 135). These duelsoccur primarily from ages 8 to 14 years, effectively bridgingjuvenility <strong>and</strong> adolescence.In one sense, juvenility is analogous to infancy. Whereasthe linguistic knowledge <strong>and</strong> structure gained in infancywill help to satisfy the informational needs of childhood,juvenility provides opportunities to achieve the persuasive<strong>and</strong> attractive use of speech, <strong>and</strong> the ability to fluentlymanipulate elaborate <strong>and</strong> socially appropriate utterances,that will be valued in adolescence.2.7. Stage 4: AdolescenceIn humans, uniquely, there is a distinct skeletal growthspurt in both sexes <strong>and</strong> in almost all skeletal elements ofthe body after several years of gently decreasing juvenilegrowth. The onset of this spurt, along with puberty orgonadarche (an event of the neuroendocrine system)marks the onset of adolescence (Bogin 1999b). Neuroendocrinologicalchanges differentially affect the vocaltract <strong>and</strong> fundamental frequency of the two sexes, femalesrevealing negligible changes, males displaying a significantincrease in tract length <strong>and</strong> decrease in frequency, withfurther drops in the transition from adolescence to adulthood(Fitch & Giedd 1999; Lieberman et al. 2001; Pedersenet al. 1986; 1990; Vuorenkoski et al. 1978). The critical variableis testosterone, which increases the length <strong>and</strong> massof the vocal folds, lowers the fundamental frequency, <strong>and</strong>alters the vibratory characteristics of the vocal folds(Abitol et al. 1999; Beckford et al. 1985; Titze 1989).Development of other secondary sexual characteristics<strong>and</strong> a growth spurt in height <strong>and</strong> weight also occur inadolescence. These physical changes are accompanied byintensification of preexisting friendships <strong>and</strong> the developmentof <strong>new</strong> relationships. The <strong>new</strong> affiliations, <strong>and</strong> membershipin peer groups, facilitate intimacy <strong>and</strong> mutualsupport (Whitmire 2000). Adolescence ends with theattainment of adult stature <strong>and</strong> the biosocial skillsneeded for successful reproduction. This occurs, onaverage, at about 19 years of age in women <strong>and</strong> 21 to 25years in men (Bogin 1999b; 2001).It is significant that the secondary sexual development ofboys <strong>and</strong> girls progresses differently through adolescence.Boys become fertile about two years after puberty <strong>and</strong>the onset of the adolescent growth spurt. At this time theystill look like boys, as their body hair, stature, <strong>and</strong> muscularityare still juvenile in appearance. The peak of their growthspurt (called “peak height velocity,” or PHV) is still, onaverage, one year away, <strong>and</strong> patterns of body hair do notbecome adult-like for three years, with a muscle spurt<strong>and</strong> adult stature still four years away (about age 18 onaverage). Girls, in contrast, will have completed abouthalf their breast <strong>and</strong> pubic hair development by the timeof their PHV. Menarche occurs about a year after PHV.At this point in development, girls have the outwardappearance of women, but they are not fertile. Developmentof an adult frequency of ovulation <strong>and</strong> adult size ofthe birth canal do not occur until approximately age 18(Bogin 1999b).Bogin (1999a; 1999b) has hypothesized that thesesex-specific patterns of adolescent development confer264 BEHAVIORAL AND BRAIN SCIENCES (2006) 29:3