Table 5.

Heterogeneity Analysis

A. Sub-sample 1: MSW+ at highest risk b	(1) Accept monthly talks & STI tests for random $α1 per month	(2) Accept monthly talks & tests for bargaining incentive $α2 per	(3) Accept to stay free of STIs for random incentive $α3 per quarter month	(4) Accept to stay free of STI for bargaining incentive $α4 per quarter
Conditional incentive	0.0838	0.0674	0.0470	−0.0098
	[0.0346]*	[0.0361]+	[0.0115]**	[0.0077]
Conditional incentive squared	−0.0037	−0.0071	−0.0009	0.0001
	[0.0025]	[0.0031]*	[0.0003]**	[0.0001]
Observations	184	182	185	178
Pseudo R-squared	0.120	0.170	0.135	0.088
Acceptance probability	0.734	0.791	0.757	0.809
Elasticities	0.436		−1.545
B. Sub-sample 2: Has HIV or STI c
Conditional incentive	0.1333	0.0905	0.0679	0.0390
	[0.074]+	[0.042]*	[0.019]**	[0.018]*
Conditional incentive squared	−0.0084	−0.0088	−0.0017	−0.0011
	[0.006]	[0.003]**	[0.001]**	[0.000]*
Observations	94	94	95	97
Pseudo R-squared	0.189	0.280	0.215	0.351
Acceptance probability	0.606	0.777	0.747	0.835
Elasticities	0.953		1.87
C. Sub-sample 3:Low-wealth population d
Conditional incentive	0.1734	0.0795	0.0520	−0.0049
	[0.032]**	[0.030]**	[0.009]**	[0.006]
Conditional incentive squared	−0.0140	−0.0087	−0.0014	−0.0000
	[0.003]**	[0.002]**	[0.000]**	[0.000]
Observations	266	260	266	259
Pseudo R-squared	0.129	0.188	0.165	0.106
Acceptance probability	0.729	0.804	0.793	0.834
Elasticities	0.506		2.03
D. Sub-sample 4: without “appropriate” condom use e
Conditional incentive	0.0538	0.0555	0.0328	−0.0089
	[0.032]+	[0.029]+	[0.009]**	[0.006]
Conditional incentive squared	−0.0056	−0.0073	−0.0010	−0.0000
	[0.002]*	[0.002]**	[0.000]**	[0.000]
Observations	315	312	321	313
Pseudo R-squared	0.0475	0.185	0.109	0.203
Acceptance probability	0.717	0.756	0.791	0.792
Elasticities	0.307		0.014
E. Sub-sample 5: Only those without appropriate knowledge of HIV/STI f
Conditional incentive	0.0784	0.0521	0.0323	−0.0040
	[0.019]**	[0.019]**	[0.006]**	[0.004]
Conditional incentive squared	−0.0062	−0.0068	−0.0009	−0.0001
	[0.002]**	[0.001]**	[0.000]**	[0.000]
Observations	712	709	717	712
Pseudo R-squared	0.0461	0.149	0.0647	0.153
Observed probability (WTA)	0.711	0.769	0.787	0.820
Elasticities	0.41		1.36

Notes: Table presents probit regression marginal effects. Robust standard errors in brackets:

^**p<0.01,

^*p<0.05,

⁺p<0.1

^aThe analytical subsamples consist of men who have sex with men (MSM), including male sex workers (MSW). MSM defined as a man who has had sex with another man with anal penetration within the last year. The conditional incentive offers were random offers for α1 and α3. The incentive offers for α2, and α 4 were follow-up questions: a bargaining experimental game to increase participation and/or reduce program costs by increasing the incentive offer (for those who initially said “no”) or to lower the incentive offer (for those who initially said “yes”). Conditional incentive offer amounts are expressed in US dollars (USD) of 2008. Data collection activities took place during October 1-December 15, 2008 in Mexico City; the average exchange rate was 12.97 pesos per USD [29]

^bSample population at the highest risk was defined as male sex workers or those with 5 or more sexual partners per month.

^cParticipants who declared to currently having been infected with HIV or with other sexually transmitted infection (STI).

^dLow-wealth sample were participants with a wealth index below the 25 percentile of the distribution; the wealth index was constructed using data on availability of: vehicle, own house, more than five rooms in the house, laptop and desktop computers, cable television, Internet access, and household help.

^eAppropriate condom use during the last sex act was defined as when respondent used a condom when he had a penetrative role only, or when partner used it when respondent had a receptive role only, or when both used it regardless of sexual roles.

^fCorrect knowledge (yes/no) was based on a battery of questions about HIV and sexually transmitted infections (STIs).