NATURE OF LOVE—SIMPLIFIED *

HARRY F. HARLOW AND STEPHEN J. SUOMI

University of Wisconsin

THE cloth surrogate and its wire surrogatesibling (see Figure 1) entered into scientifichistory as of 1958 (Harlow, 19S8). The

cloth surrogate was originally designed to test therelative importance of body contact in contrast toactivities associated with the breast, and the resultswere clear beyond all expectation. Body contactwas of overpowering importance by any measuretaken, even contact time, as shown in Figure 2.

However, the cloth surrogate, beyond its powerto measure the relative importance of a host ofvariables determining infant affection for themother, exhibited another surprising trait, one ofgreat independent usefulness. Even though thecloth mother was inanimate, it was able to impartto its infant such emotional security that the infantwould, in the surrogate's presence, explore a strangesituation and manipulate available physical objects(see Figure 3), or animate objects (see Figure 4).Manipulation of animate objects leads to play ifthese animate objects are age-mates, and play isthe variable of primary importance in the develop-ment of normal social, sexual, and maternal func-tions, as described by Harlow and Harlow (1965).It is obvious that surrogate mothers, which aremore docile and manipulative than real monkeymothers, have a wide range of experimental uses.

SIMPLIFIED SURROGATE

Although the original surrogates turned out tobe incredibly efficient dummy mothers, they pre-sented certain practical problems. The worst of theproblems was that of cleanliness. Infant monkeysseldom soil their real mothers' bodies, though wedo not know how this is achieved. However, in-fant monkeys soiled the bodies of the original clothsurrogates with such efficiency and enthusiasm asto present a health problem and, even worse, a

1 This research was supported by United States PublicHealth Service Grants MH-11894 and FR-0167 from theNational Institutes of Health to the University of Wis-consin Primate Laboratory and Regional Primate Re-search Center, respectively.

Requests for reprints should be sent to Harry F. Harlow,Regional Primate Research Center, University of Wiscon-sin, Madison, Wisconsin 53706.

financial problem resulting from laundering. Fur-thermore, we believed that the original cloth sur-rogate was too steeply angled and thereby rela-tively inaccessible for cuddly clinging by theneonatal monkey.

In the hope of alleviating practical problems in-herent in the original cloth surrogate, we con-structed a family of simplified surrogates. Thesimplified surrogate is mounted on a rod attached toa lead base 4 inches in diameter, angled upwardat 25°, and projected through the surrogate's bodyfor 4 inches, so that heads may be attached ifdesired. The body of the simplified surrogate isonly 6 inches long, 2| inches in diameter, andstands approximately 3 inches off the ground.Figure 5 shows an original cloth surrogate andsimplified surrogate placed side by side.

As can be seen in Figure 6, infants readily clingto these simplified surrogates of smaller body anddecreased angle of inclination. Infant monkeys dosoil the simplified surrogate, but the art and act ofsoiling is very greatly reduced. Terry cloth slip-covers can be made easily and relatively cheaply,alleviating, if not eliminating, laundry problems.Thus, the simplified surrogate is a far more practicaldummy mother than the original cloth surrogate.

SURROGATE VARIABLESLactation

Although the original surrogate papers (Harlow,1958; Harlow & Zimmermann, 1959) were writtenas if activities associated with the breast, par-ticularly nursing, were of no importance, this isdoubtlessly incorrect. There were no statisticallysignificant differences in time spent by the babieson the lactating versus nonlactating cloth surrogatesand on the lactating versus nonlactating wire sur-rogates, but the fact is that there were consistentpreferences for both the cloth and the wire lactatingsurrogates and that these tendencies held for boththe situations of time on surrogate and frequencyof surrogate preference when the infant was ex-posed to a fear stimulus. Thus, if one can accepta statistically insignificant level of confidence, con-sistently obtained from four situations, one will

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162 AMERICAN PSYCHOLOGIST

Fio. 1. Cloth and wire surrogate mothers.

properly conclude that nursing is a minor variablebut one of more than measurable importanceoperating to bind the infant to the mother.

To demonstrate experimentally that activities as-sociated with the breasts were variables of signifi-cant importance, we built two sets of differentiallycolored surrogates, tan and light blue; and usinga 2 X 2 Latin square design, we arranged a situa-tion such that the surrogate of one color lactatedand the other did not. As can be seen in Figure 7,the infants showed a consistent preference for thelactating surrogate when contact comfort was heldconstant. The importance of the lactational vari-able probably decreases with time. But at least

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CLOTH FED• CLOTH MOTHER

•---•WIRE MOTHER

WIRE FEDo - 0 CLOTH MOTHER0---0 WIRE MOTHER

23 85 105MEAN AGE

125 145 165

FIG. 2. Contact time to cloth and wire surrogate.

we had established the hard fact that hope springseternal in the human breast and even longer in thebreast, undressed.

Facial Variables

In the original surrogates we created an orna-mental face for the cloth surrogate and a simpledog face for the wire surrogate. I was workingwith few available infants and against time toprepare a presidential address for the 1958 Amer-ican Psychological Association Convention. On thebasis of sheer intuition, I was convinced that theornamental cloth-surrogate face would become astronger fear stimulus than the dog face whenfear of the unfamiliar matured in the monkeysfrom about 70 to 110 days (Harlow & Zimmermann,1959; Sackett, 1966). But since we wanted eachsurrogate to have an identifiable face and had fewinfants, we made no effort to balance faces byresorting to a feebleminded 2 X 2 Latin squaredesign.

Subsequently, we have run two brief unpublishedexperiments. We tested four rhesus infants un-familiar with surrogate faces at approximately 100days of age and found that the ornamental facewas a much stronger fear stimulus than the dog

NATURE OF LOVE—SIMPLIFIED 163

FIG. 3. Infant monkey security in presence of cloth surrogate.

FIG. 4. Infant play in presence of surrogate. FIG. S. Original surrogate and simplified surrogate.

164 AMERICAN PSYCHOLOGIST

FIG. 6. Infant clinging to simplified surrogate.

face. Clearly, the early enormous preference forthe cloth surrogate over the wire surrogate was nota function of the differential faces. Later, weraised two infants on cloth and two on wire sur-rogates, counterbalancing the ornamental and dogfaces. Here, the kind of face was a nonexistentvariable. To a baby all maternal faces are beauti-ful. A mother's face that will stop a clock willnot stop an infant.

The first surrogate mother we constructed came alittle late, or phrasing it another way, her babycame a little early. Possibly her baby was illegiti-mate. Certainly it was her first baby. In despera-tion we gave the mother a face that was nothingbut a round wooden ball, which displayed no traceof shame. To the baby monkey this featureless

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FIG. 7. Infant preference for lactating cloth surrogate.

face became beautiful, and she frequently caressedit with hands and legs, beginning around 30-40days of age. By the time the baby had reached90 days of age we had constructed an appropriateornamental cloth-mother face, and we proudlymounted it on the surrogate's body. The babytook one look and screamed. She fled to the backof the cage and cringed in autistic-type posturing.After some days of terror the infant solved themedusa-mother problem in a most ingenious manner.She revolved the face 180° so that she alwaysfaced a bare round ball! Furthermore, we couldrotate the maternal face dozens of times andwithin an hour or so the infant would turn itaround 180°. Within a week the baby resolvedher unfaceable problem once and for all. She liftedthe maternal head from the body, rolled it intothe corner, and abandoned it. No one can blamethe baby. She had lived with and loved a facelessmother, but she could not love a two-faced mother.

These data imply that an infant visually re-sponds to the earliest version of mother he en-counters, that the mother he grows accustomedto is the mother he relies upon. Subsequentchanges, especially changes introduced after matura-tion of the fear response, elicit this response withno holds barred. Comparisons of effects of baby-sitters on human infants might be made.

Body-Surface Variables

We have received many questions and complaintsconcerning the surrogate surfaces, wire and terrycloth, used in the original studies. This mountainof mail breaks down into two general categories:that wire is aversive, and that other substanceswould be equally effective if not better than terrycloth in eliciting a clinging response.

The answer to the first matter in question isprovided by observation: Wire is not an aversive

Cl CLOTHSB RAYONHI VINYL

C3 SANDPAPER

FIG. 8. Effect of surface on surrogate contact.

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NATURE OF LOVE—SIMPLIFIED

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ROCKING MOTHER0----0 STATIONARY MOTHER

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20 60 100 140 180 220 260DAY OF AGE

20 60 tOO 140 180 220 260DAY OF AGE

FIG. 9. Infant contact to stationary and rocking planes. FlG'10- Infant contact to stationary and rocking surrogates.

stimulus to neonatal monkeys, for they spend muchtime climbing on the sides of their hardware-clothcages and exploring this substance orally andtactually. A few infants have required medicaltreatment from protractedly pressing their facestoo hard and too long against the cage sides. Ob-viously, however, wire does not provide contactcomfort.

In an attempt to quantify preference of variousmaterials, an exploratory study2 was performed inwhich each of four infants was presented with achoice between surrogates covered with terry clothversus rayon, vinyl, or rough-grade sandpaper. Asshown in Figure 8, the infants demonstrated a clearpreference for the cloth surrogates, and no sig-nificant preference difference between the otherbody surfaces. An extension of this study is inprogress in which an attempt is being made tofurther quantify and rank order the preference forthese materials by giving infants equal exposuretime to all four materials.

Motion Variables

In the original two papers, we pointed out thatrocking motion, that is, proprioceptive stimulation,was a variable of more than statistical significance,particularly early in the infant's life, in bindingthe infant to the mother figure. We measured thisby comparing the time the infants spent on

2 We wish to thank Carol Furchner, who conducted thisexperiment and the described experiment in progress.

two identical planes, one rocking and one stationary(see Figure 9) and two identical cloth surrogates,one rocking and one stationary (see Figure 10).

Temperature Variables

To study another variable, temperature, wecreated some "hot mamma" surrogates. We didthis by inserting heating coils in the maternal bodiesthat raised the external surrogate body surfaceabout 10° F. In one experiment, we heatedthe surface of a wire surrogate and let four infantmacaques choose between this heated mother anda room-temperature cloth mother. The data arepresented in Figure 11. The neonatal monkeysclearly preferred the former. With increasing agethis difference decreased, and at approximately 15

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11. Infant contact to heated-wire and room-temperature cloth surrogates.

166 AMERICAN PSYCHOLOGIST

days the preference reversed. In a second experi-ment, we used two differentially colored cloth sur-rogates and heated one and not the other. Theinfants preferred the hot surrogate, but frequentlycontacted the room-temperature surrogate for con-siderable periods of time.

More recently, a series of ingenious studies onthe temperature variable has been conducted bySuomi, who created hot- and cold-running sur-rogates by adaptation of the simplified surrogate.These results are important not only for the in-formation obtained concerning the temperaturevariable but also as an illustration of the successfulexperimental use of the simplified surrogate itself.

The surrogates used in these exploratory studieswere modifications of the basic simplified surrogate,designed to get maximum personality out of theminimal mother. One of these surrogates was a"hot mamma," exuding warmth from a conventionalheating pad wrapped around the surrogate frameand completely covered by a terry cloth sheath.The other surrogate was a cold female; beneaththe terry cloth sheath was a hollow shell withinwhich her life fluid—cold water—-was continuouslycirculated. The two surrogates are illustrated inFigure 12, and to the untrained observer they look

remarkably similar. But looks can be deceiving,especially with females, and we felt that in thesesimilar-looking surrogates we had really simulatedthe two extremes of womanhood—one with a hotbody and no head, and one with a cold shoulderand no heart. Actually, this is an exaggeration,for the surface temperature of the hot surrogatewas only 7° F. above room temperature, whilethe surface temperature of the cold surrogate wasonly 5° F. below room temperature.

In a preliminary study, we raised one femaleinfant from Day IS on the warm surrogate for aperiod of four weeks. Like all good babies shequickly and completely became attached to hersource of warmth, and during this time she exhibitednot only a steadily increasing amount of surrogatecontact but also began to use the surrogate as abase for exploration (see Figure 13). At the endof this four-week period, we decided that our sub-ject had become spoiled enough and so we replacedthe warm surrogate with the cold version for oneweek. The infant noticed the switch within twominutes, responding by huddling in a comer andvocalizing piteously. Throughout the week ofbitter maternal cold, the amount of surrogate con-tact fell drastically; in general, the infant avoided

FIG. 12. Warm (left) and cold simplified surrogates.

NATURE OF LOVE—SIMPLIFIED 167

FIG. 13. Infant clinging to and exploring from warm simplified surrogate.

the surrogate in her feeding, exploratory, andsleeping behaviors. Feeling somewhat guilty, weswitched surrogates once more for a week andwere rewarded for our efforts by an almost im-mediate return to previously high levels of sur-rogate contact. Apparently, with heart-warmingheat, our infant was capable of forgiveness, evenat this tender age. At this point, we switchedthe two surrogates daily for a total two weeks, butby this time the infant had accepted the inherentfickle nature of her mothers. On the days that hersurrogate was warm, she clung tightly to its body,but on the days when the body was cold, shegenerally ignored it, thus providing an excellentexample of naive behaviorism.

With a second infant we maintained this pro-cedure but switched the surrogates, so that hespent four weeks with the cold surrogate, followedby one week with the warm, an additional weekwith the cold, and finally a two-week period inwhich the surrogates were switched daily. Thisinfant became anything but attached to the cold

surrogate during the initial four-week period, spend-ing most of his time huddling in the corner of hiscage and generally avoiding the surrogate in hisexploratory behavior (see Figure 14). In suc-ceeding weeks, even with the warm surrogate, hefailed to approach the levels of contact exhibitedby the other infant to the cold surrogate. Ap-parently, being raised with a cold mother hadchilled him to mothers in general, even those beam-ing warmth and comfort.

Two months later both infants were exposed toa severe fear stimulus in the presence of a room-temperature simplified surrogate. The warm-motherinfant responded to this stimulus by running tothe surrogate and clinging for dear life. The cold-mother infant responded by running the other wayand seeking security in a corner of the cage. Weseriously doubt that this behavioral differencecan be attributed to the sex difference of our sub-jects. Rather, this demonstration warmed ourhopes and chilled our doubts that temperaturemay be a variable of importance. More specifically,

FIG. 14. Typical infant reactions to cold simplified surrogate.

168 AMERICAN PSYCHOLOGIST

FIG. 15. The supersimplified surrogate.

it suggested that a simple linear model may notbe adequate to describe the effects of temperaturedifferences of surrogates on infant attachment. Itis clear that warmth is a variable of major import-ance, particularly in the neonate, and we hazard theguess that elevated temperature is a variable ofimportance in the operation of all the affectionalsystems: maternal, mother-infant, possibly age-mate, heterosexual, and even paternal.

PEOSPECTIVES

Recently we have simplified the surrogate motherfurther for studies in which its only function isthat of providing early social support and securityto infants. This supersimplified surrogate ismerely a board 1J inches in diameter and 10 incheslong with a scooped-out, concave trough having amaximal depth of f inch. As shown in Figure IS,the supersimplified surrogate has an angular devia-tion from the base of less than 15°, though thisangle can be increased by the experimenter at will.The standard cover for this supremely simple sur-rogate mother is a size 11, cotton athletic sock,though covers of various qualities, rayon, vinyl(which we call the "linoleum lover"), and sand-paper, have been used for experimental purposes.

Linoleum lover, with you I am throughThe course of smooth love never runs true.

This supersimplified mother is designed to at-tract and elicit clinging responses from the infantduring the first 15 days of the infant's life.

We have designed, but not yet tested, a swingingmother that will dangle from a frame about 2 inchesoff the floor and have a convex, terry cloth or cottonbody surface. Observations of real macaqueneonates and mothers indicate that the infant, notthe mother, is the primary attachment object evenwhen the mother locomotes, and that this swingingmother may also elicit infantile clasp and impartinfant security very early in life. There is nothingoriginal in this day and age about a swingerbecoming a mother, and the only new angle, ifany, is a mother becoming a swinger.

Additional findings, such as the discovery thatsix-month social isolates will learn to cling to aheated simplified surrogate, and that the presenceof a surrogate reduces clinging among infant-infantpairs, have substantiated use of the surrogatebeyond experiments for its own sake. At present,the heated simplified surrogate is being utilized asa standard apparatus in studies as varied as re-action to fear, rehabilitation of social isolates, anddevelopment of play. To date, additional researchutilizing the cold version of the simplified surrogatehas been far more limited, possibly because unusedwater faucets are harder to obtain than emptyelectrical outlets. But this represents a method-ological, not a theoretical problem, and doubtlesslysolutions will soon be forthcoming.

It is obvious that the surrogate mother at thispoint is not merely a historical showpiece. Unlikethe proverbial old soldier, it is far from fadingaway. Instead, as in the past, it continues tofoster not only new infants but new ideas.

REFERENCES

HARLOW, H. F. The nature of love. American Psycholo-gist, 1958, 13, 673-685.

HARLOW, H. F., & HARLOW, M. K. The affectional systems.In A. M. Schrier, H. F. Harlow, & F. Stollnitz (Eds.),Behavior of nonhuman primates. Vol. 2. New York:Academic Press, 1965.

HARLOW, H. F., & ZIMMERMANN, R. R. Affectional re-sponses in the infant monkey. Science, 1959, 130, 421-432.

SACKETT, G. P. Monkeys reared in visual isolation withpictures as visual input: Evidence for an innate releasingmechanism. Science, 1966, 154, 1468-1472.