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Mathematical Population Studies

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Do Men and Women Have the Same Average Number of Lifetime

Partners?

MARC ARTZROUNIa; EVA DEUCHERTb

a Department of Mathematics, University of Pau, France b Swiss Institute for Empirical Economic

Research, University of St. Gallen, Switzerland

Online publication date: 05 November 2010

To cite this Article ARTZROUNI, MARC and DEUCHERT, EVA(2010) 'Do Men and Women Have the Same Average

Number of Lifetime Partners?', Mathematical Population Studies, 17: 4, 242 — 256

To link to this Article: DOI: 10.1080/08898480.2010.514853

URL: http://dx.doi.org/10.1080/08898480.2010.514853

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Do Men and Women Have the Same Average

Number of Lifetime Partners?

Marc Artzrouni

Department of Mathematics, University of Pau, France

Eva Deuchert

Swiss Institute for Empirical Economic Research, University of

St. Gallen, Switzerland

It is generally thought that for sake of consistency men and women must have the

same average number of lifetime partners. However, this is not the case in general.

When men have younger partners, women enter sexual relationships more quickly

than men and have a higher number of lifetime partners. A male dominant model

applied to UK data on the male rate of entry into a sexual relationship and

the male partnership formation function shows that in a stationary population

(zero growth rate) women have 9.1%more partners than men. In a stable

population with an intrinsic growth rate of 2%and a larger but still plausible

difference between the ages of partners, women have 24.6%more partners than

men. Given that in sex surveys men report more partners than women, the result-

ing bias in estimated numbers of partners may therefore be larger than previously

thought.

Keywords: lifetime partners; male dominant model; stable population; stationary

population; United Kingdom

1. INTRODUCTION

To gain a better understanding of the dynamics of sexually trans-

mitted diseases, we rely on accurate data about sexual behavior. The

average number of lifetime sexual partners (LSPs) is a key parameter

(May and Anderson, 1992). Surveys of sexual behavior document

substantial discrepancies between men’s and women’s self-reported

numbers of LSPs (Buve

´et al., 2001; Laumann et al., 1994; Smith,

Address correspondence to Marc Artzrouni, Department of Mathematics, University

of Pau (BP 1155), 64013 Pau Cedex, France. E-mail: Marc.Artzrouni@univ-pau.fr

Mathematical Population Studies, 17:242–256, 2010

Copyright #Taylor & Francis Group, LLC

ISSN: 0889-8480 print=1547-724X online

DOI: 10.1080/08898480.2010.514853

242

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1992). Men and women report 10.8 and 6.2 partners, respectively, in

Quebec; 10.1 and 4.4 in France; and 11.5 and 5.0 in the United States

(Brisson et al., 1999). The UK’s National Survey of Sexual Attitudes

and Lifestyles II yields 12.7 lifetime partners for men and 6.5 for

women.

It is believed that this sex discrepancy is mathematically impossible

because with male and female populations of equal size, average num-

bers of LSPs should be the same for both sexes. Researchers have

ascribed this difference to various forms of response or sample selec-

tion bias which can result from misreporting or from sexual contacts

outside the sampled populations, notably with sex workers (Brown

and Sinclair, 1990; Catania, 1999; Brewer et al., 2000).

The equality of numbers of partners for men and women depends on

the definition. We will see that the numbers of entries into relation-

ship at one point in time—added over all age groups—must be the

same for both sexes. However, the average number of LSPs estimated

in sex surveys is the average over all ages aof the cumulated number

of relationships experienced up to age a. We show that when men

choose younger partners, the average number of LSPs is higher for

women than for men because women have an earlier sexual debut

than men.

In section 2 we discuss an illustrative example with two age groups.

We then use a continuous-time male dominant version of the model to

derive male and female average numbers of LSPs, which are consist-

ent but not necessarily equal. We give conditions under which the

female average will be greater than the male one. In section 3 we fit

male entry into relationship and male partnership functions to data

from the UK’s National Survey of Sexual Attitudes and Lifestyles II

(NATSAL II). In section 4 we discuss the implications of our results,

particularly with regard to the inconsistencies found in data on sexual

behavior.

2. MODEL

2.1. Two-Age-Group Example

We consider a stationary two-age-group population consisting of 200

young men and women and 100 old men and women (Figure 1). We

assume that men choose the total number of partners they desire

(male dominant model). Each young man chooses one woman, abbrevi-

ated as ‘‘pt’’ for partner. Each older man chooses two younger women

and one older one. The number of relationships experienced by an indi-

vidual of sex mduring the k-th period (m¼1 (women) or m¼2 (men))

Number of Lifetime Partners of Men and Women 243

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is k

m

(k). The numbers of partners chosen by young and old men are

k

2

(1) ¼1 and k

2

(2) ¼3. The imputed numbers of partners per woman

are k

1

(1) ¼(1 200 þ2100)=200 ¼2 and k

1

(2) ¼1. The average

number of entries into relationship at each period by men and women

is the same at 1.67.

If we let C

m

(k) be the cumulative number of partners of sex mup

to period k, then for men we have C

2

(1) ¼1 and C

2

(2) ¼4. This

means an average male number A

2

of LSPs equal to 2. The fact that

women have more partners when they are young than when they

are old translates into cumulative numbers in the first and second

age groups equal to C

1

(1) ¼2 and C

1

(2) ¼3, respectively. The female

average number of LSPs is equal to A

1

¼2.33, a figure that is 17%

higher than A

2

¼2 obtained for men. The higher figure for women

comes from the fact that women start accumulating partners at a

younger age.

In Figure 1, the fact that men choose younger partners was miti-

gated by the young age structure. If, however, both age groups have

200 individuals, then with other parameters remaining unchanged,

the imputed numbers of partners for women are 3 and 1 for the

two age groups instead of 2 and 1 previously. The relative scarcity of

young women means they have more partners and results in 2.5 and

3.5 LSPs for men and women, respectively. This translates into a

40%greater number for women.

FIGURE 1 Two-age-group example to illustrate the difference between male

and female numbers of sexual partners. Average numbers of partners during

one period are the same (1.67). The cumulated number of partners up to the

second age group is higher for men (4) than for women (3). The mean of the

lifetime numbers over the two periods is the mean number of lifetime sexual

partners (LSPs) estimated in sex surveys and is higher for women (2.33) than

for men (2).

244 M. Artzrouni and E. Deuchert

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This simple example shows that when men choose younger women,

women have more lifetime partners than men. In order to extend the

results to a realistic population, we describe a continuous-time version

of this model and prove a theoretical result when men always choose

younger partners.

2.2. Average Lifetime Number of Sexual Partners

We consider an age-structured two-sex population defined by female

and male densities N

k

(x) at age x(k¼1 for women, 2 for men). The

rates of entry into relationship are k

k

(x), k¼1, 2. This means that

individuals of sex kwhose age is in the interval (x,xþdx) establish

N

k

(x)k

k

(x)dx heterosexual partnerships. These partnerships can be

instantaneous with sex workers or repeated with lifetime partners.

The period lifetime number of sexual partners C

k

(a) for an individ-

ual of sex kand age ais the integral of k

k

(x)uptoagea:

CkðaÞ¼

def:Za

0

kkðxÞdx;k¼1;2:ð1Þ

The corresponding average numbers A

k

of LSPs in the population aged

between m

1

and m

2

are now:

Ak¼

def:Rm2

m1CkðsÞNkðsÞds

Rm2

m1NkðsÞds ¼Zm2

m1

ðRs

0kkðxÞdxÞNkðsÞds

Rm2

m1NkðsÞds ;k¼1;2:ð2Þ

2.3. Men/Women Consistency Condition

We postulate a male dominant model characterized by a male partner

acquisition function k

2

(x). We derive the male average number A

2

of

LSPs from Eq. (2). In order to calculate a consistent female average

number A

1

of LSPs, we first define the conditional probability density

function f

1

(uja) of the age uof a male partner given a woman’s age a

(‘‘female partnership formation function’’). Similarly, f

2

(aju) is the con-

ditional probability density function of the age aof a female partner

given a man’s age u(‘‘male partnership formation function’’). The fact

that the densities of new partnerships between women aged aand

men aged uare the same from the male and female perspectives

means that:

k1ðaÞN1ðaÞf1ðujaÞ¼k2ðuÞN2ðuÞf2ðajuÞ:ð3Þ

We bear in mind that f

1

(uja) and f

2

(aju) are density functions

(Rx

0f2ðajuÞda ¼1¼Rx

0f1ðujaÞdu where xis the maximum age in the

Number of Lifetime Partners of Men and Women 245

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population). We integrate both sides of Eq. (3) over uto obtain:

k1ðaÞN1ðaÞ¼Zx

0

k2ðuÞN2ðuÞf2ðajuÞdu:ð4Þ

The imputed female rate of entry into relationship is:

k1ðaÞ¼Rx

0k2ðuÞN2ðuÞf2ðajuÞdu

N1ðaÞ:ð5Þ

In order to calculate numbers of partners in a cohort, we assume

that male and female populations are stable. This means that both

populations have a constant age distribution and grow at the same

exponential rate. The densities N

2

(u) and N

1

(a) in Eq. (5) are then of

the form N

2

(u)exp(rt) and N

1

(a)exp(rt), where ris the intrinsic growth

rate and tis time. The male number A

2

of Eq. (2) and the imputed

female rate of entry into relationship k

1

(a) of Eq. (5) do not change over

time because exp(rt) cancels out in the numerator and the denomi-

nator. The cumulation

C1ðsÞ¼Za¼s

a¼0

k1ðaÞda ¼Za¼s

a¼0Rx

0k2ðuÞN2ðuÞf2ðajuÞdu

N1ðaÞda ð6Þ

is, for all female cohorts, the total number of relationships experienced

up to age s. We use this C

1

(s) to express the female total number of

LSPs of Eq. (2):

A1¼Rs¼m2

s¼m1Ra¼s

a¼0Rx

0k2ðuÞN2ðuÞf2ðajuÞdu

N1ðaÞda

N1ðsÞds

Rm2

m1N1ðsÞds :ð7Þ

This expression shows that a consistent female average number of

LSPs is expressed in terms of the male rate k

2

(u) and the male partner

formation function f

2

(aju). Moreover, integrating both sides of Eq. (4)

over ayields:

Zx

0

k1ðaÞN1ðaÞda ¼Zx

0

k2ðuÞN2ðuÞdu;ð8Þ

which shows that as required the total number of male and female new

relationships are equal. This is the common 1.67 of the introductory

two-age-group example. Also, Eq. (3) is satisfied with an imputed

female density f

1

(uja) equal to:

f1ðujaÞ¼

def:k2ðuÞN2ðuÞf2ðajuÞ

k1ðaÞN1ðaÞ¼k2ðuÞN2ðuÞf2ðajuÞ

Rx

0k2ðuÞN2ðuÞf2ðajuÞdu :ð9Þ

246 M. Artzrouni and E. Deuchert

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2.4. Theoretical Result

In the two-age group examples, men choosing younger women meant

that women had more LSPs than men (A

1

>A

2

). The role of the age

structure in this result is highlighted in Proposition 1, which provides

sufficient conditions for A

1

to be higher than A

2

.

Proposition 1. Under the assumptions:

.a

1

: the female and male population densities are equal on the

interval I ¼(m

1

,m

2

)(0, x): N

1

(x) ¼N

2

(x) for x 2I;

.a

2

: a man has no partner older than himself, or f

2

(aju) ¼0 for a >u

(therefore Ru

0f2ðajuÞda ¼1);

.a

3

: the populations during partnership formation years (ages u such

that k

2

(u) >0) are increasing functions of age: N

1

(u)=N

1

(a) >1 when

u>a,

we have A

1

>A

2

: the average number of LSPs calculated for ages

between m

1

and m

2

is higher for women than for men.

Proof. Eq. (2) shows that with equal male and female population den-

sities N

k

(x) for x2I(Assumption a

1

) the female average A

1

is greater

than the male average A

2

if C

1

(s)>C

2

(s) for any s2I. This means that

the total number of relationships up to age sis larger for women than

for men. To prove that C

1

(s)>C

2

(s):

C1ðsÞ¼Zs

0Rx

0k2ðuÞN2ðuÞf2ðajuÞdu

N1ðaÞda ðEq:ð6ÞÞ ð10Þ

¼Zs

0Rx

0k2ðuÞN1ðuÞf2ðajuÞdu

N1ðaÞda ða1:N2ðuÞ¼N1ðuÞÞ ð11Þ

¼Zx

0

k2ðuÞZs

0

N1ðuÞ

N1ðaÞf2ðajuÞda

du ðintegral exchangeÞð12Þ

>Zs

0

k2ðuÞZs

0

N1ðuÞ

N1ðaÞf2ðajuÞda

du ðutaken to sonlyÞð13Þ

¼Zs

0

k2ðuÞZu

0

N1ðuÞ

N1ðaÞf2ðajuÞda

du ða2;usÞð14Þ

>Zs

0

k2ðuÞZu

0

f2ðajuÞda

du ða3:N1ðuÞ=N1ðaÞ>1Þð15Þ

¼Zs

0

k2ðuÞdu ¼C2ðsÞða2:Zu

0

f2ðajuÞda ¼1Þ:ð16Þ

This shows that C

1

(s)>C

2

(s) and therefore A

1

>A

2

.

Number of Lifetime Partners of Men and Women 247

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Assumption a

1

is realistic because male and female population sizes

are usually close to each other until 50, an age at which the rate of

entries into relationship k

2

(u) becomes close to 0. Assumption a

2

takes

the male preference for younger partners to its extreme by assuming

that men never have an older partner.

Assumption a

3

states that the population is a decreasing function of

age at least during the partnership formation years, which can happen

in a stable population with a negative growth rate. These conditions

are sufficient to ensure that A

1

>A

2

. We will see in the numerical

applications that they are not necessary.

3. APPLICATION TO UK DATA

3.1. Background

We propose functional forms for the male parameters k

2

(u) and f

2

(aju).

These forms will be fitted to UK data and then used to obtain consist-

ent (but different) male and female averages A

1

and A

2

for a range of

scenarios concerning the growth rate of the population and the male

partnership function f

2

(aju). We will find that A

1

>A

2

in all cases even

though the conditions a

1

,a

2

, and a

3

of Proposition (1) are not always

satisfied: a

1

is only approximately true because male and female mor-

tality rates are slightly different; a

2

is not satisfied because the data

fitting will yield a density f

2

(aju) which does not drop to 0 for a>u

(some men have older partners); a

3

is not satisfied when the intrinsic

growth rate is positive (the population then decreases with age).

We use data from the UK’s National Survey of Sexual Attitudes and

Lifestyles II to estimate k

2

(u) and f

2

(aju). NATSAL II is a multistage

stratified random survey of 12,110 men and women (ages 16–44) who

were living in private households in Great Britain in 2000–2001. The

survey collected information on the total numbers of partners and

new partners over different time periods (during respondents lives,

the last five years, the last year, the last three months, and the last

four weeks). A detailed description of the survey design is in Erens

et al. (2001).

3.2. Male Rate k

2

(u) of Entry into Sexual Relationship

We model the cumulated number of partners C

2

(u) for men aged uas a

Gompertz-type function of the form

C2ðuÞ¼

def:p1expð expðp2ðup3ÞÞÞ;pk>0;k¼1;2;3;ð17Þ

248 M. Artzrouni and E. Deuchert

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where p¼(p

1

p

2

p

3

) is a vector of positive parameters: p

1

is the com-

pleted number of partners for u!1;p

2

is a measure of skewness,

and p

3

is the age at which k

2

(u) reaches its maximum. The derivative

of C

2

(u) is the male rate of entry into relationship:

k2ðuÞ¼

def:C0

2ðuÞ¼p1p2expðp2ðup3ÞÞexpðexpðp2ðup3ÞÞÞ:ð18Þ

We estimate the parameters using numbers of new partners in the

last year reported by men. Using Stata’s nonlinear least-square

routine we obtained the estimate:

^

pp ¼ð21:19 0:23 20:75Þ:

SE :ð0:90Þð0:02Þð0:24Þð19Þ

of p. Figure 2 shows that the resulting function k

2

(u) captures the

unimodal pattern in the male rate of entry into relationship, and the

rapid decline during the 20 s and 30 s.

3.3. Modeled Male Partnership Formation Function f

2

(aju)

We model f

2

(aju) as a three-parameter family of conditional Weibull

density functions of the form:

FIGURE 2 Reported average number of new partners during the last year, by

age uof man (grey bars) and corresponding fitted male partner acquisition

rate k

2

(u) of Eq. (18).

Number of Lifetime Partners of Men and Women 249

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f2ðajuÞ¼

0ifacðuÞ

aðuÞ

bðuÞ

acðuÞ

bðuÞ

aðuÞ1exp acðuÞ

bðuÞ

aðuÞ

if a>cðuÞ;

8

<

:

ð20Þ

where the shape, scale, and location parameters a(u), b(u), and c(u)are

functions of the man’s age u. This family of densities can be reparame-

terized in terms of any pair of age-specific low and high percentiles

P

L

(u) and P

H

(u) (with 0 <L<H<100) (Marks, 2005).

Given an age-specific location parameter c(u) (minimum age of

partner), the shape and scale parameters a(u) and b(u) of the density

in Eq. (20) are:

aðuÞ¼

ln lnð1H

100Þ

lnð1L

100Þ

ln PHðuÞcðuÞ

PLðuÞcðuÞ

ð21Þ

and

bðuÞ¼PHðuÞcðuÞ

ln 1

1H

100

1=aðuÞ:ð22Þ

Because a quantile regression cannot be used to estimate the

zero-th percentile c(u) we approximate c(u) with the first percentile

P

1

(u). The NATSAL II datasets show that the first, 30th, and 70th per-

centiles P

1

(u), P

30

(u), and P

70

(u) are well approximated by linear func-

tions of the age uof the man (Figure 3). Using Stata’s quantile

regression routine to calculate the coefficients we obtain:

cðuÞ’P1ðuÞ¼12:38 þ0:12u

SE :ð1:40Þð0:06Þð23Þ

P30ðuÞ¼6:57 þ0:57u

SE :ð0:25Þð0:01Þð24Þ

P70ðuÞ¼2:76 þ0:88u

SE :ð0:36Þð0:01Þ:ð25Þ

Figure 4 represents the observed and fitted male densities f

2

(aju)of

Eq. (20) for three different ages of men. It shows a good fit for ages 20

and 30 for which the sample sizes were 228 and 127, respectively. For

age 40 there are only 46 men in the sample, which explains why the fit

is not that good.

250 M. Artzrouni and E. Deuchert

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In this baseline scenario the 0th, 30th, and 70th percentiles at age

15 are 14 years and 2 months, 15 years and 1 month, and 16 years.

This means that for a young man aged 15 the minimum age of his

FIGURE 4 Observed and fitted conditional density function f(aju) of the age a

of the partner for men aged u¼20, 30, 40.

FIGURE 3 Observed and fitted linear relationships between a man’s age and

the first, 30th and 70th percentile for the partner’s age (Eq. (23)–(25)).

Number of Lifetime Partners of Men and Women 251

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partners is 14 years and 2 months; 30%of his partners are under 15

years and 1 month; 70%of the partners are under 16. These three per-

centiles are 18 years and 5 months, 35 years, and 47 years and 9

months for men aged 50.

In order to assess the sensitivity of the average ages of partners we

will consider an ‘‘Alternative 20%lower age of female partner’’ scen-

ario. The 0th, 30th, and 70th percentiles 14 years and 2 months, 15

years and 1 month, and 16 years are the same as before. In addition

the three percentiles at age u¼50 are 20%lower than the values 18

years and 5 months, 35 years, and 47 years and 9 months obtained

for u¼50 in the baseline scenario. The linear equations become:

P1ðuÞ¼13:95 þ0:015u;P30ðuÞ¼9:58 þ0:37u;P70 ðuÞ¼6:77 þ0:61u:

ð26Þ

3.4. Average Numbers of Lifetime Sexual Partners

Stable female and male population densities N

k

(x,t) at age xand time t

are of the form:

N1ðx;tÞ¼berterx skðxÞ;N2ðx;tÞ¼SRB:berterxskðxÞ;ð27Þ

where bis a positive constant, rthe intrinsic growth rate, s

k

(x) the

probability of surviving to age xfor sex k; SRB, the sex ratio at birth,

is set to 1.05, the value most commonly found in human populations.

We use recent UK data for the female and male survival rates s

1

(x)

and s

2

(x). The averages A

k

are calculated between ages m

1

¼15 and

m

2

¼50. Cumulative numbers of partners C

k

(50) at 50 and averages

A

k

are calculated for six scenarios obtained by combining:

.three intrinsic growth rates requal to 2%,0%, and 2%. (The stable

populations with r¼0%are rough approximations of the structure

of the UK population in the mid 2000s.)

.the ‘‘Baseline’’ and the ‘‘Alternative 20%lower age of female part-

ner’’ scenarios described in Section 3.3.

The modeled male cumulative number of partners C

2

(x) as well as

the imputed female cumulative numbers of partners C

1

(x) (which

depend on the scenario and on the intrinsic growth rate) are shown

in Figure 5. The asymptotic values of these functions are the com-

pleted C

k

(50) given in Table 1. The numbers A

k

of LSPs in Table 1

are the integrals of the cumulative functions weighted by the popu-

lation structures [Eq. (2) and (7)].

252 M. Artzrouni and E. Deuchert

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Because of the male dominance men choose partners independently

of the age structure and scenario. This means in particular that the

completed number of partners C

2

(50) for men is the same (21.2) for

both scenarios and all intrinsic growth rates (Table 1).

TABLE 1 Sensitivity Analysis of Cumulated and Average Numbers of

Partners to Different Intrinsic Growth Rates for Two Scenarios Concerning

the Modeled Male Partnership Formation Function f

2

(aju)

Completed C

k

(50) Average A

k

Int. growth rate C

1

(50) C

2

(50) %diff. A

1

A

2

%diff.

1. Baseline UK partnership formation function

2%22.4 21.2 6.0 18.3 16.3 12.1

0%21.7 21.2 2.4 16.7 15.3 9.1

þ2%21.0 21.2 0.6 15.2 14.2 6.6

2. Alternative 20%lower age of female partner

2%23.4 21.2 10.4 20.3 16.3 24.6

0%21.7 21.2 2.5 18.1 15.3 18.4

þ2%20.2 21.2 4.3 16.2 14.2 13.6

Averages A

k

(k¼1, 2 for women, men, respectively) are calculated over the interval

(m

1

,m

2

)¼(15, 50).

FIGURE 5 Modeled male cumulative number of partners C

2

(x) (Eq. (17),

thick solid line) with imputed female cumulative numbers C

1

(x) for baseline

and alternative scenarios and three intrinsic growth rates, designated by

the thin lines.

Number of Lifetime Partners of Men and Women 253

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Figure 5 shows that, for both scenarios and all growth rates, women

accumulate partners more quickly than men do. As expected, the dif-

ference is accentuated with the Alternative scenario for which the age

difference is larger. In each scenario, the difference between men and

women is accentuated with the negative growth rate because the rela-

tively scarce young women in a declining population have more part-

ners (thick solid line for men versus two dashed lines for women

corresponding to the two scenarios).

When r¼0%and r¼2%then for both scenarios the added effect of

the partnership formation function and of the age structure results in

female functions C

1

(x) that remain larger than the male C

2

(x) for all x.

When r¼0%the female average number A

1

of LSPs is then 9.1%and

18.4%larger than the male value, for the Baseline and Alternative

scenarios, respectively. When r¼2%, the relative shortage of

younger women exacerbates the differences which reach 12.1%and

24.6%for the two scenarios (Table 1).

When r¼2%the female functions C

1

(x) remain smaller than the

male C

2

(x) except at the end of the period of sexual activity where

C

1

(50) is 21.0 and 20.2 for the Baseline and Alternative scenarios,

respectively. These numbers are slightly smaller than the male 21.2,

while the female average number A

1

of LSPs is 6.6%and 13.6%higher

than the male A

2

, for the Baseline and Alternative scenarios, respect-

ively. The younger age structure more than offsets the fact that the

cumulative male number of partners overtakes the female one at the

end of the period of sexual activity.

4. CONCLUSION

Reported data on numbers of LSPs obtained from the NATSAL II sur-

vey yield estimates of 12.7 and 6.5 partners for UK men and women

ages 15–45. Using only reported rates of partner acquisitions and a

stationary population, we obtained consistent baseline average num-

bers of LSPs equal to 15.3 and 16.7 for men and women, respectively

(Table 1). This 9.1%difference, applied to the reported male number

12.7, which is assumed accurate, translates into an average female

number of LSPs equal to 12.7 1.091 ¼13.9. The fact that this number

is higher than the male number of 12.7 makes the 6.5 partners

reported by women appear even more biased.

The reason for the discrepancy between the reported and imputed

female numbers is apparent if one compares the reported age specific

female rate of entry into relationship obtained from the NATSAL II

with the imputed (consistent) k

1

(x) of Eq. (5) (Figure 6, Baseline

254 M. Artzrouni and E. Deuchert

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scenario, r¼0). Figure 6 shows that if reports on men are accurate,

then women either underreport the number of partners they have

had in the last year or are not representative of the partners reported

by men. Sexual contacts with sex workers reported by men might not

be represented in the survey.

The NATSAL II data were also fitted to the female dominant ver-

sion of the model. The Weibull and Gompertz functions fit the data

for women on partnership formation and acquisition as well as it fits

the data for men (details not shown). With female data assumed accu-

rate, the same baseline scenario as above yields a female number of

6.5, which is equal to the reported number. The consistent male num-

ber of LSPs is 4.8. The fact that this number is smaller than the female

number of 6.5 makes the 17.3 partners reported by men appear even

more biased.

Although our model cannot reveal the origin of the men=women dis-

crepancy in reported numbers of LSPs, it demonstrates that claims on

the accuracy of the reported numbers should not be premised on a

theoretical men=women equality. In the common case of men having

younger partners, women have a higher number of LSPs than men.

The fact that women report fewer partners than men means that

the bias is more severe than previously thought and that more must

be done to understand its origin.

FIGURE 6 Reported (bars) and imputed (k

1

(x) of Eq. (5), solid line) rates of

entry into relationship for women. The fitted male rate of entry into relation-

ship plotted in Figure 2 is given in dotted lines.

Number of Lifetime Partners of Men and Women 255

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